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Full text of "Bulletin of the British Museum (Natural History), Mineralogy"

PETROLOGY OF LORD HOWE 
ISLAND, PART i : THE YOUNGER 

VOLCANICS 



p. M. GAME 



BULLETIN OF 
THE BRITISH MUSEUM (NATURAL HISTORY) 
MINERALOGY Vol. 3 No. 5 

LONDON : 1970 



PETROLOGY OF 

LORD HOWE ISLAND 

PART I : THE YOUNGER VOLCANICS 



BY 

PHILIP MALCOLM GAME 



pp. 221-284 ; 21 text-figures ; plates 10-15 



BULLETIN OF 

THE BRITISH MUSEUM (NATURAL HISTORY) 

MINERALOGY Vol. 2 No. 5 

LONDON : 1970 



THE BULLETIN OF THE BRITISH MUSEUM 

(natural history), instituted in 1949, is 
issued in five series corresponding to the Departments 
of the Museum, and an Historical series. 

Parts will appear at irregular intervals as they 
become ready. Volumes will contain about three or 
four hundred pages, and will not necessarily be com- 
pleted within one calendar year. 

In 1965 a separate supplementary series of longer 
papers was instituted, numbered serially for each 
Department. 

This paper is Vol. 2, No. 5 of the Mineralogy 
series. The abbreviated titles of periodicals cited 
follow those of the World List of Scientific Periodicals. 



World List abbreviation 
Bull. Br. Mus. nat. Hist. (Miner.) 



Trustees of the British Museum (Natural History), 1970 



TRUSTEES OF 
THE BRITISH MUSEUM (NATURAL HISTORY) 

Issued 20 April, 1970 Price £2 5s. 

(£2.25) 



PETROLOGY OF LORD HOWE ISLAND 
PART I : THE YOUNGER VOLCANICS 

Bv P. M. GAME 



CONTENTS 



I. 
II. 

III. 



IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XI. 



id) 



Introduction ......... 

Petrology of the Gower-Lidgbird Lavas and the younger 

DYKES .......... 

Mineralogy of the Gower-Lidgbird Lavas and the younger 

DYKES .......... 

(a) Plagioclase ........ 

(6) Order-disorder in plagioclase phenocrysts 

Pyroxenes ........ 

Olivines ........ 

Iron ores ....... 

Bulk chemical analyses ....... 

Relation between composition and distance from ridge crest 
Notes on the older series 
Ball's Pyramid 
Age of the island . 
Summary and conclusions 
Acknowledgements . 
References 

Appendix : (i) Petrography of the Gower-Lidgbird lavas 
(2) Petrography of analysed dyke rocks . 

SYNOPSIS 



Page 
224 

225 

231 
231 

237 
242 

251 

253 
256 
265 
267 
268 
268 
271 
274 
274 
277 
284 



Lord Howe Island is the only island on the Lord Howe Rise, a major physiographic feature 
of the south west Pacific, running for 2,000 miles from South Island, New Zealand to New 
Caledonia. Lord Howe and Norfolk Island, which lies on the crest of a parallel ridge 300 miles 
to the east, are the only islands in this wide area (2,000 miles from east to west), lying between 
east Australia and the Tonga Trench, and bounded to the north by New Caledonia and to the 
south by New Zealand. Lord Howe Island volcanics are of typical alkali basalt composition, 
but show considerable textural and mineralogical variations. At least three major eruptive 
periods are represented, the youngest, from radiogenic dating, being of mid-Pliocene age. 
Vulcanism began at least as early as the Mid -Tertiary and no major change in its character is 
indicated throughout the period of activity (considered to be at least 20 m.y.). The present 
island represents only about one percent of its original volume. The upper flows of the youngest 
volcanic sequence (the Gower-Lidgbird volcanics) show some differentiation towards a trachy- 
tic end product. The mean percentages of the significant oxides almost conform to the peak 
frequencies of these same oxides in islands of typical oceanic character, as recorded by Chayes 
(1964). There is, however, in each instance a slight but systematic displacement towards the 



224 PETROLOGY OF LORD HOWE ISLAND, PART I 

peak frequencies for circum-oceanic basalts. This may be connected with the setting of Lord 
Howe Island in a sea with a " Mediterranean " type of sea floor structure, and a crustal thickness 
estimated at 20 km. The degree of silica undersaturation for the Lord Howe Island basalts 
and for their most differentiated product accords with that of islands displaced a similar distance 
(just over 200 miles) from the crest of the Mid-Atlantic Ridge. Fifteen analyses of Lord Howe 
Island rocks are presented ; together with two analyses of their pyroxenes, microprobe analyses 
of the ores, and optical analyses of plagioclases, pyroxenes and olivines. The order-disorder 
degrees in the plagioclase phenocrysts, as indicated by the most sensitive optical parameters are 
discussed. 

I. INTRODUCTION 

Lord Howe Island lies 300 miles east of the coast of New South Wales, and about 
260 miles west of the crest of the Lord Howe Rise which is a major physiographic 
feature of the south west Pacific. Lord Howe Island and Norfolk Island, which Hes 
on the crest of a parallel ridge 300 miles further east, are the only islands in this great 
area of " Mediterranean " type sea-bed crust between eastern Australia and the deep 
Kermadec-Tonga Trench 2,000 miles to the east. New Zealand borders the southern 
margin of this sea and New Caledonia and Fiji the northern (Text-fig. i). 

The isolated positions of Lord Howe and Norfolk Islands on submarine ridges 
having crustal thicknesses of about 20 km. (Officer, 1955) and lying off a continental 
shelf from which they are separated by only a minor trench (the Ulladulla Trench) 
divides them, in the oceanographical sense, from the multitude of typical " oceanic " 
islands on the eastern side of the deep Kermadec-Tonga Trench (with depths to 
35,000 ft) and crustal thicknesses of the order of 5-10 km. Lord Howe Island, which 
lies closer to the mainland and probably has the greater vertical exposures was chosen 
for prior investigation. It is hoped to compare the petrology of the two islands 
at a later date. The present paper deals exclusively with the petrology of Lord 
Howe Island. 

Only brief descriptions of the volcanic rocks of Lord Howe Island have been pub- 
hshed (David, 1889, and Standard, 1963). There is moreover a difference of opinion 
between these two investigators as to the relative ages of the two main volcanic 
formations. 

The only account of Norfolk Island petrology is that pubUshed by Speight (1913) 
more than 50 years ago, the main interest of which is the suggestion that enclosures 
of plutonic origin may occur in the tuffs forming the neighbouring Phillip Island. 

The main objectives of the petrological examination of Lord Howe Island were : 

1. To determine the number and type of volcanic units of which the island is 
composed. 

2. To investigate the mineralogical, textural and chemical variations in the fresh 
volcanic rocks. 

3. To determine the extent and trend of differentiation in the fresh rocks. 

4. To find chemical matches from other provinces for the Lord Howe Island 
volcanic rocks. 

5. To measure the absolute and relative ages of the volcanic formations. 

6. To find out whether any petrological changes occurred throughout the period 
of vulcanicity. 



PETROLOGY OF LORD HOWE ISLAND. PART I 225 

7. To determine whether Lord Howe Island volcanics belong to the " Oceanic " 
or " Circum-oceanic " petrological types, as distinguished by Chayes (1964). 

8. To relate the degree of silica undersaturation of the Lord Howe Island rocks 
to the distance of the island from the crest of its submarine ridge. (In the Atlantic 
and East Pacific such a relation has been found to exist) . 

9. To determine the composition and variation of the four main mineral phases, 
namely plagioclase, pyroxene, olivine and iron ore ; in particular to investigate the 
thermal history of the plagioclase phenocrysts. 

No arrangements could be made for dredging rocks from the island's submarine 
slopes and, consequently, the objectives of the expedition could only be partly 
realised. 

In the limited time available for fieldwork (18 days in August 1965) it was im- 
possible to examine systematically all the volcanic units, and consequently the work 
was mainly concentrated on the fresh Gower-Lidgbird volcanics and the younger 
dyke swarm. Some of the agglomerate exposures, the more altered groups of lavas 
and the older, altered dykes were briefly visited. The following groups of volcanic 
rocks were recognised on the island : 

1. A fresh series of horizontal flows, a little over 2,800 ft in thickness forming 
Mounts Lidgbird and Gower, at the south end of the island. (Gower-Lidgbird 
series) (PL 13, i and 2). 

2. A dyke swarm cutting the altered lavas in the northern half of the island, but 
not penetrating the Gower-Lidgbird lavas. 

3. A rather altered series of flows, also horizontal, occupying the northern half 
of the island. The maximum exposure is about 700 ft in Malabar hill, at the NE 
tip of the island (" Horizontal altered series "). 

4. A partly altered series of flows, dipping at 25° to 30° to the SW (i.e. under the 
horizontal lavas of Mt Lidgbird) and outcropping between Mutton Bird Point and 
Red Point. About 1000 ft are exposed (" Inclined altered series "). 

5. Agglomerates in the lower levels of the inclined altered series; these agglo- 
merates are particularly well exposed at the south end of Blinkenthorpe beach (PI. 
14, 2) and at the south end of Salmon beach. 

6. A series of basic sheets and dykes cutting the inclined altered series and par- 
ticularly abundant in the agglomerates. 



II. PETROLOGY OF THE GOWER LIDGBIRD LAVAS 
AND THE YOUNGER DYKES 

About 2840 ft of horizontal flows are perfectly exposed in the cliffs of Mts Gower 
and Lidgbird (PI. 13). They are however difficult of access, especially in the 
upper part where the north ridge of Mt Gower (connecting it to Mt Lidgbird) is the 
only route. The flows vary greatly in thickness and include several massive horizons 
50-60 ft thick with prismatic jointing. A layer of bole forms the upper part of many 
flows, proclaiming their subaerial weathering. The lowest 250 ft of the Gower- 
Lidgbird lavas are mainly obscured by scree and can only be collected from the base 
of Mt Gower, which there was insufficient time to visit. However, the remain- 



226 PETROLOGY OF LORD HOWE ISLAND, PART I 

ing 2500 ft of these lavas was sampled throughout its thickness, a total of forty 
specimens being obtained. The main features of this group are summarized in 
appendix i. 

Petrology. With the exception of the cumulate types (from five different heights 
in the series) and an altered trachytic lava (from an inclined thin sheet near the top 
of the series) the great majority of these flows belong to the alkali-basalt province, 
although considerable textural and petrological variations occur. Lavas from most 
levels are porphyritic with 30% of phenocrysts in some instances (BM.1966, P5, 30). 
Olivine is the commonest phenocryst. Near the centre of the series (at about the 
1500 ft level) there is a group of basalts with very sparse, small phenocrysts. In some 
lavas (BM.1966, P5, 20, 21, and 22) phenocrysts are virtually absent. Five separate 
cumulate flows were collected. These are of the oceanite or oceanite-ankaramite 
type and have up to nearly 60% of olivine plus pyroxene cumulate crystals (BM. 
1966, P5, 33 from 720 ft above sea level). Glass is fairly common (18 of the 39 flows) 
although it only occurs in relative abundance in three flows. Flow structure is well 
developed in a quarter of the lavas and present in just over half (PL 12, 2). The 
fabric is mainly intergranular, although a well developed sub-ophitic fabric occurs 
in the intercumulate material of three of the five cumulate types. OHvine (mostly 
iddingsitized) is present in the groundmasses of most of the lavas and biotite occurs 
in the groundmass of one lava (BM.1966, P5, 131). A zeolite (chabazite) and car- 
bonate occur particularly in the groups of lavas from near the centre of the sequence. 
The iron ore consists of ilmenite and/or an ulvospinel-magnetite. 

In the highest flow (BM.1966, P5, i) a xenolith of hypersthene, calcic plagioclase 
and a diopsidic augite occurs, the whole being surrounded by a rim of granular 
olivine. No other xenoliths were noted. In this flow the finely crystallized plagio- 
clase has been partly albitized. 

The dyke swarm. The dykes that cut the altered, older volcanic flows are well 
exposed in the cliffs and wave-cut platforms along the northern half of the island. 
They are usually vertical or steeply dipping ; the majority strike towards Mt Lidg- 
bird and are roughly radial about this focus. Thickness varies from a few inches 
to about 15 ft, but the majority do not exceed 6 ft. No composite dykes were 
observed. The mean spacing of the dykes is about 20 yards, but locally the swarm 
is very much denser. At three localities (cliffs west of the Old Gulch, cliffs SW of 
North beach and on Rabbit Island PL 15, i) the dykes are so dense that only rem- 
nants of the older lavas can be discerned between them. The more normal spacing 
is seen in the dykes along the remainder of the North Ridge, where, in the almost 
vertical cliffs, 80 dykes can be counted spanning a distance of a mile. These are sub- 
parallel and perpendicular to the cliff face ; unfortunately sampling is here impossible. 
Text-fig. 2 shows the positions and trends of all the dykes examined. It is important 
to note that the dykes do not extend into or intersect the lavas that build Mounts 
Gower and Lidgbird. (There are perfect exposures on the great cliff faces of these 
two mountains.) Consequently the relative age of the Gower-Lidgbird series is 
younger than that of the " horizontal altered series " that forms the northern part of 
Lord Howe Island, and confirms the opinion expressed by David (1889). 

Petrology of dyke swarm. The great majority of dykes have a simple mineralogical 



PETROLOGY OF LORD HOWE ISLAND, PART I 227 

composition ; in these olivine is rare or absent and plagioclase, diopsidic pyroxene 
and ore are the essential constituents. Glass, usually chocolate or khaki coloured is 
abundant in some dykes and is sometimes associated with analcite. Carbonate 
occurs as a late stage mineral but before the deuteric period. A feature of many 
dykes is the infiUing of spherical vesicles by minerals of hydrothermal or deuteric 
origin, mainly soda plagioclase, carbonate, chlorite, ore and (devitrified) glass 
(PI. 10, i). The carbonate, if present, is always at the centre and is the last mineral 
to crystallize. The plagioclase occurs as acicular crystals. Exceptionally the vesi- 
cles are 3 mm. in diameter, but the average size is i mm. They are especially 
abundant in some dykes from Blinkenthorpe beach. Middle beach, and the quarry 
near Windy Point near the lagoon. They occur less abundantly in the North beach, 
Ned's beach and Rabbit Island dykes. In the Admiralty Islets and in the few dykes 
carrying olivine, vesicles are absent. Carbonate of composition Ca3iFe48Mg2i 
(from analysis) and appearing to be of primary origin is common in many dyke rocks 
and in some (e.g. BM.1966, P5, 83) from Blinkenthorpe beach it forms 15% of the 
mode and 12% of the norm. Other dykes from the same locality and also from 
Rabbit Island carry considerable amounts of this " primary " iron-rich carbonate. 
In nearly all dyke rocks examined both feldspar and pyroxene of the groundmass 
show, by bent crystals and uneven extinction, signs of strain. The groundmass 
plagioclases are often zoned. The great majority of dykes (43 out of 52) are por- 
phyritic. Plagioclase is by far the most abundant and widely occurring phenocryst. 
In one third of the dykes examined plagioclase phenocrysts are abundant, but in the 
eight dykes carrying porphyritic olivine, plagioclase phenocrysts are absent in five 
and scarce and small in the remaining three. The porphyritic plagioclase shows a 
strong tendency to form glomero-groups (PI. 10, 3). Pyroxene phenocrysts are less 
common and less abundant in the dykes than are plagioclase phenocrysts. Both 
tend to be idiomorphic, but the plagioclase always precedes the pyroxene in the intra- 
telluric period of crystallization. 

In the rare olivine basalt dykes (8 out of the total of 52 examined), olivine occurs 
almost entirely as phenocrysts up to 3 mm. in length. In the groundmass it is absent 
or sparse. The phenocrystal olivines of the dykes usually show incipient alteration to 
serpentine, and are never iddingsitized as they commonly are in the flows. Although, 
as previously stated, olivine bearing dykes form but a small percentage of aU dykes 
examined, they are more abundant at some localities than at others. Thus, at 
North beach, of the 20 dykes examined along the shore line west of the beach only 
two (BM.1966, P5, 61 and 71) contain relatively conspicuous (but not abundant) 
olivine phenocrysts. Three other dykes at this locality show rare olivine phenocrysts 
but in the remaining dykes olivine is absent. On the Admiralty Islets, however, 
two of the few dykes examined contained plentiful olivine phenocrysts ; 22|% of 
modal phenocrysts occur in a dyke (BM.1966, P5, 102) from this locaUty. A dyke 
from Middle beach (BM.1966, P5, 88) contained just over 30% of modal olivine 
whilst at Ned's beach an oHvine bearing dyke is intersected by, and therefore older 
than a plagioclase basalt dyke, devoid of olivine. Consequently it is possible that the 
olivine bearing dykes preceded the non olivine types in order of intrusion. The 
petrography of five analysed dyke rocks is described in Appendix 2. 




00 

ON 



a 

•a 
a 



C 
a! 
> 



n! 
V 

U 

o 

X) 

a 



<D 

o 



nJ 
X! 
O 



s 

+-> 






230 



PETROLOGY OF LORD HOWE ISLAND, PART I 

o 

ADMIRALTY 

c5d>J\^ islets 

XJO 







D (diogrammatic)80 dyl" 



'*'' D (d iogrammalic) 80 d^kes in the cliff sectioP 

% 
"6 



Blinkenthorpe Beach 



Mutton Bird Islond 



Mutton Bird Point 
A 




Intermediate .MSOlder dipping 
Hill 

Boot 



lovo^SQCuf by dykes 



D Dykes (Strike as shown) 
A Agglomerate 



■ ?.-^.^S CORAL 



LAND OVER 150 meters 



LORD HOWE ISLAND 
1 Km 



Fig. 2. Map of Lord Howe Island, showing localities and strikes of dykes examined. 



PETROLOGY OF LORD HOWE ISLAND. PART I 231 

III. MINERALOGY OF THE GOWER LIDGBIRD LAVAS 
AND THE YOUNGER DYKES 

(fl) Plagioclase of the lavas and dykes 

The composition of the plagioclase was determined on a Leitz 4-axis universal 
stage. For the larger crystals (phenocrysts and the largest groundmass crystals) 
a full Fedorow technique was used. The twin axis was derived from the positions 
of aai, ^|8i, and yyi, where a, ^ and y are the poles of the indicatrix axes in one 
member of the twin and ai, ^1, and yi, are the corresponding poles in the second 
twin member. This technique avoids the use of any morphological plane and gives 
less scatter in the composition values than do determinations made with reference to 
a composition or cleavage plane. Having thus derived the twin-axis, the composi- 
tion of the plagioclase and its twin-law can be obtained from the angles between this 
axis and a, ^ and y, using the curves published by Slemmons (1962) for the various 
twin laws. The total number of plagioclase phenocrysts thus determined was 138 
from the lavas and 49 from the dykes. 

The great majority of the groundmass crystals were too small to permit the deter- 
mination of the indicatrix in each twin member. For these small crystals the compo- 
sition plane (almost invariably 010) was used as a reference direction and the angle 
between its pole and the pole of either a or y was measured. This value was again 
referred to the appropriate curve on Slemmons' diagram to determine composition (but 
not the twin law) . This method was found to be more precise and less tedious than the 
"maximum symmetrical extinction angle" method. Optic axial angles were mea- 
sured wherever possible, but were not used in the determination of composition since 
the relationship between 2V and composition is not known with precision in volcanic 
plagioclases (see p. 239). For each flow in the Gower-Lidgbird sequence, and for each 
selected dyke rock the compositions of five phenocrysts and at least five groundmass 
crystals were determined. The ranges in compositions are thus directly comparable. 

The results of the plagioclase determinations for both lavas and dykes are plotted 
in Text-fig. 3. This shows no dramatic nor immediately obvious compositional 
trend. Nevertheless three separate rhythmic cycles can be distinguished for the 
phenocrystal plagioclase of the Gower-Lidgbird succession ; these are shown in the 
diagram by inclined lines, trending from relatively more calcic to relatively more 
sodic plagioclase. In each instance the highest member of the sequence has about 
10% more Ab than the lowest member. From near the base to near the top of the 
Gower-Lidgbird lavas the superimposed effect of these rhythms is to produce a 
phenocrystal plagioclase (An 55) in the highest levels which has about 10% more 
Ab than the plagioclase (An 65) near the base. 

In the lavas there is usually no compositional gap between the phenocrysts and 
the groundmass plagioclase. In some lavas it is impossible to draw a definite 
boundary, i.e. to distinguish two generations in spite of a considerable composition 
range from the smallest to the largest crystals. Thus a flow (BM.1966, P5, 40) from 
300 feet above sea level gave the following plagioclase compositions in passing from 
the smallest to the largest grains : An 44, 47^ 48^ 51, 54, 54. 57, 57, 57, 57^, 60, 61. 
The accumulate lava types show similar effects. An oceanite from 2200 ft above sea 



232 



Meters Feet 



762-5 2500 



610 2000 



457-5 1500 

Height above 
sea level 



305 1000 



152-5 500 



30 




BM 1966. P5. 



•4 



■5 



-■7 
-■8 



-11 

13 
15 

4-16 

17 
18 



20 
21 



24 



--26 



-. 27 
• 28 

31 
■1- 32 



■33 

-34 

■35 

36 



39 

±38 

■-37 

-40 



90 



LOCALITY 

North beach 

Blinkenthorpe 
bay 

Middle beach 
Quarry lagoon 

Ned's beach 

Rabbit island __ 

Admiralty 
islets 

Ball's pyramid "" 



PETROLOGY OF LORD HOWE ISLAND, PART I 233 

B. DYKE ROCKS BM 1966,P5. 



I ^ 

I- J-SQ,^J^^ w9LPi-"i _ — ^ _ - 



h 






I 1 



Zoned grains 
I- ----1 I 1 



I 1 , 1 

I 1 

I 1 i-i 

I HI 1 

I I 



30 40 SO 60 

Mol. Percentage of Anorthite 



70 



80 



56,66 

83 

93 

100 
76,77 
96 
102 

- 105 



90 



I 1 Groundmass plogioclase 

>==- = . ^ _ ^Groundmass plogioclase Approx. det'n. 

I 1 Phenocrysfol &Xenocrysfal plogioclase 

C * Cumulate 

Fig. 3. Compositions of plagioclases from (a) the Gower-Lidgbird sequence and (b) selected 

dyke rocks from eight different localities. 



level (BM.1966, P5, 8) showed progressive increase, by roughly equal instalments, 
from the smallest grains (An 44) to the largest (An 67). 

In the lavas xenocrysts (An 73 to 78) occur only in the lowest flows, and even here 
they are uncommon. By contrast, the larger plagioclase crystals of the dykes con- 
tain a plentiful array of bytownite xenocrysts (An 71-84) in addition to the rarer 
phenocrysts (An 58-68). 

The plagioclase from the dykes, in contrast to that from the lavas, shows a com- 
positional gap between phenocrysts and groundmass which sometimes exceeds 10% 
An. A dyke (BM.1966, P5, 96) from Rabbit Island for example has groundmass 
plagioclase of composition An 46-54, while its phenocrystal plagioclase has a compo- 
sition of An 67. 

Some dykes contain several types of larger plagioclase crystals. An example of the 
complexity that may occur is given by the plagioclases from a dyke 6 ft wide (BM. 
1966, P5, 56) outcropping just west of North beach. The plagioclases from a single 
section of this dyke included : 

(i) Normal phenocrysts in the form of clear, idiomorphic, prismatic crystals hav- 
ing a mean composition of An 66. 



234 



PETROLOGY OF LORD HOWE ISLAND, PART I 



(2) Subidiomorphic and somewhat corroded grains, often in glomero-groups, 
containing abundant inclusions of pyroxene, ore and glass, giving them a " spongy " 
or " checkerboard " appearance (PI. 10, 4). These plagioclases also show some degree 
of preferred orientation. Many are normally zoned having bytownite cores (An 70- 
77), proceeding outwards with some discontinuities, to exteriors of andesine (An 44- 
48). A few grains of apparently uniform composition have very narrow sodic 
rims. Pericline and Ala twins are confined to these spongy plagioclases, being ab- 
sent from the remainder which have albite or Carlsbad twins. 



COMPOSITION 

PLANE(010)albite twin 

An 75 




An 82 



An 65 



Fig. 



An54 at inner margin 
grading to An 40 at 
outer. 

Diagrammatic sketch of crystal from dyke at North beach, showing oscillatory 

zoning. 



(3) Clear idiomorphic, lath-shaped or prismatic crystals (presumably xenoliths) 
of bytownite (An 73-81) often occurring in glomero-groups. They are either un- 
zoned or have narrow exterior rims or terminations with an average composition of 
An 65. These crystals show, dominantly, Carlsbad twinning. 

(4) Crystals with oscillatory zoning ; these occur singly. Several idiomorphic 
examples occur, of which one is illustrated in Text-fig. 4. The composition of each 
shell is shown in the diagram. The extreme range in composition in this crystal is 
from An 40 to An 82. In PI. 11, i a discontinuously zoned plagioclase (two zones 
only) is shown. The example just quoted illustrates the variety and complexity 
of the so called phenocrysts from the dyke rocks. 

Twin laws. The distribution of twin laws in the phenocrysts of the lavas and the 



PETROLOGY OF LORD HOWE ISLAND, PART I 235 

dykes is given in Table I. In both the lavas and in the dykes, albite, Carlsbad or 
albite-Carlsbad twins comprise nine-tenths or more of all twins that occur. The 
percentage of twins recorded as albite-Carlsbad tends to be higher than it probably 
should be because very thin lamellae often separate the two main twin members. 
Many twins recorded as albite-Carlsbad are probably albite plus Carlsbad twins. 
These two types (albite and Carlsbad) account for over three quarters of all twins, 
and there is no essential difference in twin laws shown by plagioclases from the lavas 
on the one hand and from the dykes on the other. 



Table i 

Distribution of Twin Laws in Plagioclases in Lavas 

AND Dykes 



Twin law 


% in lavas 


(69) 


% in dykes (52) 


albite 


39 




28 


Carlsbad 


3oi 




43 


albite-Carlsbad 


20 




23 


pericline 


6 




4 


ala 


I* 




2 


Manebach 


3 




— 



Groundmass plagiodase. Text-fig. 3 shows that above 2200 ft the groundmass 
plagioclases of the Gower-Lidgbird lavas follow the trend of the phenocrysts towards 
enrichment in albite, but below this level there are no definite changes. Similarly 
the plagioclases from the groundmass of the dykes show no definite trend. Its 
mean composition from the eight different localities all lie within 5% An of the overall 
mean (An 52) and thus show no significant variation with locaUty. Text-fig. 5 
shows the distribution of composition of the unzoned, or very slightly zoned, ground- 
mass plagioclases of the lavas and dykes. There is a close correspondence between 
both the mean compositions and the degree of scatter. The two means differ by 
less than 2% in An content, and this difference is partly due to the inclusion of 
groundmass plagiodase from the upper flows, which as already mentioned, is some- 
what more sodic than the bulk of the groundmass plagiodase from the main part 
of this sequence. 

Zoning is common in the groundmass plagiodase of the dykes and also in the 
smaller plagiodase grains of the accumulate type of lava, in which crystals with a 
core of composition approximating to An 50 have exteriors of An 30-35. In those 
lavas in which the two generations of plagiodase are sharply contrasted, zoning 
of the groundmass plagiodase is both less common and less intense ; differences in 
composition between cores and exteriors are of the order of An 10 to 15. The twin 
laws of the groundmass plagioclases are usually impossible to determine, but the 
composition plane was found to be 010 in 97% of all measured grains and 001 in the 
remaining 3%. 



236 



PETROLOGY OF LORD HOWE ISLAND, PART I 



20 . 



15 



10 



(A 

c 
o 

o 

> 

0) 

lA 

Ji 

o 



15 



4) 

EIO 

z 



5 , 



mean 



a) Gower Lidgbird lavas 



41 44 47 50 53 56 59 

Mol /o of Anorthite 



mean 



b) Dykes ( 8 Localities) 



44 47 50 53 56 59 62 

Mol /o of Anorthite 
Fig. 5. Distribution of composition of plagioclase in groundmass. 



PETROLOGY OF LORD HOWE ISLAND, PART I 237 

{b) Order-disorder in plagioclase phenocrysts 

In measuring the composition of the plagioclase phenocrysts in the Gower-Lidgbird 
series and the dyke rocks by the most accurate optical techniques possible (i.e. 
without recourse to any morphological planes the data accumulated are considered 
sufficient to warrant drawing orientation curves for two twin laws and a mean optic 
axial angle curve ; these curves are for volcanic plagioclases in the An 50 to An 85 
composition range. The object was to investigate the degree of ordering of the 
lattice and also, in the case of 2V, to try to produce a reUable curve (or " band ") 
in this region where, as Marfunin (1966) so truly states, " data are contradictory 
and inadequate ". 

Text-fig. 6 shows the value of a'^ twin axis and /S^ twin axis for plagioclases having 
albite-Carlsbad twins (T.A. = l[ooi] in (010)) and for plagioclases having albite 
twins (T.A. = 1(010)). The curves published by Slemmons (1962) are also shown. 
Although Slemmons' curves for the limiting states of albite-Carlsbad twinned plagio- 
clases (completely disordered high-temperature and completely ordered low- 
temperature plagioclases) are separated by less than 2° in the region An 50 to An 70, 
the plot for the Lord Howe labradorites shows that this is a real difference that can be 
measured. AU points for these feldspars are above Slemmons' low temperature 
curve. They show an almost even distribution on each side of his high temperature 
curve. There is however, in the An 60 to An 70 region a very slight, but probably 
significant tendency to a state not completely disordered in a few grains. Thus, 
in the total of 45 points, aU except three are less than 1° from the high temperature 
curve. The three exceptions — all exactly 1° from the curve — lie on the low tempera- 
ture side of it. However the evidence is far too slender to try to derive any quanti- 
tative value for the " degree of ordering ". For plagioclases having albite twins and 
more calcic than about An 70 the optical differences are less than 1°, and it is im- 
possible to derive any information about order-disorder. But, from An 70 to An 50, 
the optical differences become progressively more marked At An 50 the difference 
between the angles for the extreme states is 6|° and therefore easily detectable. For 
crystals with albite twins, the standard deviation of /S^loio (the angle that expresses 
the degree of order-disorder) as determined from 20 sets of 5 repeated measurements 
on the same crystal is o°-38. Consequently differences exceeding 1° (nearly three 
times the standard deviation) in this angle cannot be ascribed to experimental 
error, but are probably the result of different degrees of ordering in different crystals. 
The actual crystals measured, with compositions from about An 46 to An 54, are 
symmetrically disposed about the high temperature curve. It must be concluded 
(admittedly on the basis of only four determinations) that these plagioclases which 
are the most sodic phenocrysts in the Lord Howe volcanics are disordered high tem- 
perature types. From An 55 to An 67 there are 18 determinations ; of these 4 lie 
below the high temperature curve and 12 above (on the low temperature side) : 
6 of these 12 are displaced from the high temperature curve by 1° or more. In this 
limited compositional range, therefore, there is sufficient evidence upon which to 
base an " intermediate " curve (I-I in the figure). 

If, following Uruno (1962) and Slemmons (1962) we try to evaluate order-disorder 

B 



238 



PETROLOGY OF LORD HOWE ISLAND, PART I 



45 



40 



35 



30' 




An 60 



ALBITE CARLSBAD TWINS 




Anso 



An 80 



-1 — I — ■ — I I p I 



50" 



55' 



o^'^TA 



60" 



I ■ ■» » ■— 

65" 



70" 



6^ 



60 



55" . 



I I I I 
75" 



An 50 



i-"" 



An 70 



An 60 



< 



An 80 




Anso An70 



ALBITE TWINS 



Aneo 



Anso 



75" 



60° 



iP^ 



^'^TA 



70° 



80" 



Fig. 6. Relation between a'^ twin axis, ^S^ twin axis and composition for plagioclases from 
the Gower-Lidgbird lavas and from the dyke rocks. The solid lines are Slemmons' curves 
for high temperature plagioclases. The broken lines are Slemmons' curves for low 
temperature plagioclases. The dotted line I-I is the " intermediary index of ordering " 



curve. 



PETROLOGY OF LORD HOWE ISLAND, PART I 239 

by using an index of for the most disordered form and an index of i-o for the most 
ordered form, and assign intermediate indices to intermediate types according to 
their positions relative to the and i-o forms, we obtain for the I-I curve, in Text-fig. 
6 an intermediate index of about 0-2, since this curve lies about one-fifth of the total 
distance between the high temperature (disordered) and low temperature (ordered) 
curves. This figure (0-2) agrees with that given by Uruno in his abstract (plagio- 
clases in fresh volcanic rocks — 0-2) although he quotes a figure of 0-28 in a later table 
(Table 8). It also agrees exactly with the value obtained by Slemmons for his 
"intermediary" index obtained from X-ray measurements [2^(i3i)-20(i3i)] for 
plagioclases in the An 60 to An 70 compositional range. However, since there are, 
in all, three points lying 1° or more below the high temperature curve drawn by 
Slemmons there may be some evidence that this curve does not represent a state of 
complete disorder but should be displaced slightly downwards. 

Text-fig. 6 therefore suggests that, although many plagioclases from the Lord 
Howe fresh volcanic rocks are completely disordered high temperature types, there 
are a minority that show some degree of ordering. This finding is in agreement 
with that of Uruno (1962) who notes that whilst the majority of plagioclase pheno- 
crysts in volcanic and hypabyssal rocks show low ordering, there are some that are 
more ordered. 

optic axial angles. Text-fig. 7 shows the values of 2V for those plagioclases (a) 
in which both optic axes were within the field of view and (b) the composition of 
which was regarded as having been accurately determined (triangles of error for the 
twin axis small or non-existent). The figure includes values of 2V published by 
van der Kaaden (1951) and a few by Kohler (1942) for volcanic plagioclases. Of the 
total of 104 values, one half lie within a belt extending i\° on each side of the " best 
fitting " curve. An angular error of i^° corresponds to a compositional error of 
about 5% in the An 50 and An 75 regions where the curve is " flat " but to an error 
of only about i^% An in the An 60- An 70 region where the curve is relatively steep. 
Thus the 50% chance of estimating the composition from 2V for volcanic plagio- 
clases with compositions between An 50 and An 85 varies from i| to 5% An. 15 per 
cent, of points lie 3^° or more from the curve. There is, therefore, a 15% chance of 
making an error of from 6% to 7% An in determinations derived from 2V measure- 
ments. Consequently 2V is of only secondary value as a guide to composition. 

The 2V values obtained by van der Kaaden, by Kohler and by the author for vol- 
canic plagioclases in this range (50 to 85 An) are in mutual agreement, permitting the 
drawing of the curve shown in the figure. But this mean curve is completely at 
variance with two curves derived by Smith (1958) for low temperature and high 
temperature plagioclases within these compositional limits. The discrepancy can be 
seen by reference to Marfunin's (1966, fig. 56) diagram in which van der Kaaden's 
curve is contrasted with the curves published by Smith. As Marfunin remarks 
" low temperature curves of Smith almost coincide with high-temperature curves of 
van der Kaaden ". Smith's high temperature curve was based on only three points 
in the An 60 to An 80 range, and these were obtained by heating plagioclases consi- 
dered (from X-ray evidence) to be " the lowest temperature plagioclases so far 
found ". The paucity of points on the high temperature curve might explain the 



240 



PETROLOGY OF LORD HOWE ISLAND, PART I 



\ 



\ O O 
\ • 

• \ 

• \« 
\ 



\ 



\ 



\ 



f 



\ o 
o 

\ 

o 
» o 



\ 



\ 

o o 



\ o 
\ 



\ 



o 
o 



0« 



o 
o 



•• • 

• o • 

o \ 
o a«« 

°^* 



IE 

o 

c 
o 



0) 

o> 
o 

c 

V 
u 

a> 
a 



3 
U 

o 



O 

o 



•o 



bo 



\ 



\ o 



d 



•\0 



o 
00 

o 

o 



^AT 









ee 



CD 



O V 

Ml rO 
to r- 

3 TO 

-S c 

m en 

u 3 
(^-^ 

D Oj 
U > 

•3^ 



O 
en 



en 

Cm 



en 
en 

O 

'So 



,is <u o 



a 

CM 
y o 

eti j3 
— * f 



C en 
o en 



> 
u 



HI B 

o ^ 

en ^^5 
p <D 

'3 ^ 

^; '^ 
t^ iH 

. o 

O (-1 



N 



Si 
■■o 



0\ 



PETROLOGY OF LORD HOWE ISLAND, PART I 241 

discrepancies noted here, but Smith's low temperature curve is based on eight values 
(An 55 to An 85) and it seems impossible to explain the difference between this curve 
and those obtained by other investigators. Uruno (1962) has published values of 2V 
for plagioclases with an " ordering degree " of 0-6 to 0-8 (presumably not volcanic 
plagioclases) and the curve obtained from his values does not differ greatly from 
Smith's low temperature curve 

The average width of the " zone of scatter " for 2V for volcanic plagioclases of 
compositions from 50 An to 85 An, as measured by Koher (1942), van der Kaaden 
(1951) and the author is from 6° to 8°. This is about the same as the angular varia- 
tions published by Smith between his " completely low temperature " and " com- 
pletely high temperature " curves. All the facts indicate that Smith's limiting 
curves should lie further apart, and that their trend cannot be accepted for volcanic 
plagioclase. The standard deviation for 2V determinations, as derived from 20 sets 
of 5 repeated measurements (resetting the optic axial plane for each set) is 0-33°. 
Consequently the maximum experimental error (three times the standard deviation) 
is 1°. Similar considerations show that the maximum error in the determination of 
composition is 2% An in the An 50-65 range and 3% An in the An 65-85 range. 
The probable errors are less than one-third of these amounts. Therefore, a scatter 
of 6° to 8° in 2V is far outside the limits of experimental error. The " phenocrysts " 
of basalts must have had widely differing thermal histories, producing different 
degrees of order-disorder. This is consistent with the fact that in BM.1966, P5, 56 
(see p. 233) at least four differing types of plagioclase were found in a single dyke. 
Variations in order-disorder degree are not confined to the phenocrysts. Two pairs 
of xenocrysts (one pair from the same section) the members of each pair having 
virtually identical compositions (An 77 for one pair and An 81 for the other) have 2V 
values differing by 2-5°. It is, of course, possible that a varying proportion of other 
elements could partly account for the differences noted in 2V for plagioclases of the 
same essential composition. The elements most suspect are Ba, Sr, and Fe. A 
microprobe analysis was made to determine the amounts of these elements in two 
plagioclases from adjacent flows (BM.1966, P5, 37 and 38) at the base of the Gower- 
Lidgbird series, having identical compositions (An 62) but optic axial angles differing 
by 5|°. The results were : 

Table II 

Values of 2V and the percentage of Ba, Sr, Fe and Ca for Plagio- 
clases FROM two adjacent Gower-Lidgbird lavas 



BM.1966, P5 


2V 


Ba 


Sr 


Fe + 2 
Fe + 3 


Ca 


37 
38 


8l-2° 

867° 


0-39 
0-09 


0-I2 

o-i8 


071 
0-52 


IO-I2 

9-75 



It is impossible at present to evaluate the effect of these differences in the distribu- 
tion of these minor elements, but it is worth noting that differences, especially in the 
Ba content, exist. Determinations on synthetic plagioclases incorporating known 



242 PETROLOGY OF LORD HOWE ISLAND, PART I 

amounts of these elements might indicate their influence. Meanwhile there is a 
need for many more measurements of volcanic plagioclase optic axial angles, after 
precise determinations of their composition. Slemmons' (1962) diagrams indicate 
that this parameter shows greater variations than do the X-ray reflection differences, 
based on measurements by Smith (1958) and by Smith and Yoder (1956) for two 
pairs of strong peaks that are sensitive to structural changes, namely 20(131)- 
2^(131) and 20(i3i)-20(22o). The only conclusion that can at present be drawn for 
basaltic plagioclase phenocrysts is that they show marked differences in their order- 
disorder relationships which indicates that their rates of cooling must have varied 
within wide limits, suggesting different depths of derivation. Smith (appendix to 
Hess, i960) suggests that useful information might be gained about the structural 
complexities of plagioclases by separate determinations of the " optical properties 
of extensive groups of samples of comparable thermal history ". But the trouble 
is that, in basaltic rocks, it is impossible to recognize crystals of " comparable 
thermal history ". Such rocks contain (even in a single section) groups of crystals 
which have had different rates of cooling. 

(c) Pyroxenes of the lavas and dykes 

Pyroxene is present in the groundmass of all the flows of the Gower-Lidgbird 
sequence and in the dyke rocks. The modal proportion of pyroxene is usually 30 to 
35%. Much of the groundmass pyroxene, especially in the dykes, shows evidence 
of strain. Pyroxene occurs as phenocrysts in about 50% of the flows and 70% of 
the dykes, but it is only abundant in about a quarter of the flows and 10% of the 
dykes. In the dykes the pyroxene phenocrysts are usually sparse and small and 
much less plentiful than the plagioclase phenocrysts, around which they are often 
moulded, being of later crystallization. In both dykes and flows pyroxene tends to 
form glomero-porphyritic groups (PI. 12, 4). In some of the lavas larger, single 
pyroxenes occur, up to i cm. in diameter. Most pyroxene phenocrysts from both 
flows and dykes tend to be idiomorphic (PI. 12, 3) ; twinning on 100 is common. 

Although the relations between composition and optical properties are insufficiently 
well known to derive absolute compositions, measurement of 2V and the refractive 
index provides a useful means of detecting any compositional variations throughout 
the sequence. These parameters were therefore determined for phenocrysts, and, 
wherever possible, for groundmass pyroxenes of both flows and dykes. 

The refractive index (y) was determined by the single variation method, using 
sodium light and iodonapthalene. The grains were placed in this liquid on a hot 
stage which was connected, in parallel, to an Abbe refractometer. Hot water was 
circulated through this system, and the temperature was varied until an exact match 
was obtained. The refractive index was then obtained, directly, from the refracto- 
meter reading. For the 2V measurements a Leitz 4-axis universal stage was used, 
and only those determinations were accepted in which direct observation of both axes 
was possible. y^C was also determined where possible, on twinned crystals using the 
method of Turner (1942). 

The results of the optical determinations are given in Table III. It is evident from 
this table that whilst the composition of the two pyroxene generations differ there 



PETROLOGY OF LORD HOWE ISLAND, PART I 



243 



Table III 

Refractive Indices (y), Optic Axial Angles and y'^C Values for Clinopyroxenes of the 

GOWER-LlDGBIRD LaVAS AND OF THE YOUNGER DYKE ROCKS 

Lavas 







RI(y) 




2V (mean 


value) 




BM.I966, P5 


Height 
a.s.l. 


A 




A 




i,AP 


r 

Phenocrysts 


G-mass 


f 

Phenocrysts 


G-mass 


1 / *-' 


4 


2400 ft 


1-718 


— 


53-6° 


— 


— 


7 


2300 ft 


— 


1-737 


— 


49-2° 


— 


8 


2200 ft 


1-726 


— 


53-7° 


— 


— 


13 


1950 ft 


1-725 


— 


53-0'' 


— 


— 


17 


1750 ft 


1-724 


— 


53-2° 


— 


— 


18 


1 700 ft 


1-720 


— • 


53-6° 


— 


45° 


24 


1300 ft 


— 


1-733 


52.7° 


— 


— 


31 


950 ft 


1-722 


— 


— 


— ■ 


— 


32 


900 ft 


1-722 


— 


53-0° 


— 


45° 


33 


700 ft 


1-722 


— 


53-4° 


— 


— 


35 


600 ft 


1-720 


— 


— 


50-8° 


42r 


39 


330 ft 


1-718 


— 


— 


— 


— 


40 


300 ft 


1-724 

RI{y) 


Dykes, 


2V (mean 


51-0° 
value) 




BM.1966, P5 


Locality 


A 




A 




yAC 


r 

Phenocrysts 


G-mass 


r 

Phenocrysts 


G-mass 


82-83 


Blinkenthorpe 
beach 


— 


1-733 


53-2° 


48-2° 


— 


95-97 


Rabbit Island 




1-735 


52-2° 




44° 

(microphenocryst) 

45i° (g-mass) 


55. 59, 66, 


North beach 


1-717, 


1-730 


52-1° 


50-2° 


45i° (g-mass) 


70, 71 




1-719 










102 


Admiralty 
Islets 


1-719 





52-5° 


— 


45° (g-mass) 


76, 79 


Ned's beach 


— 


1-734 


52-0° 


— 


— 


98, 100, lOI 


Quarry 
near lagoon 




1-733 


52-9° 


48-6° 


43r 

(microphenocryst) 

46° 
(microphenocryst) 


89,91.92,93 


Middle beach 


1-720 


1-732 


52-3° 


48-5° 


47° (g-mass) 



are no variations that exceed experimental error in either groundmass or phenocrysts. 
Table IV shows the mean optical values for both lavas and dykes combined. 

The range in values of the y index for phenocrysts and for groundmass pyroxenes 
is given in Text -fig. 8 whilst Text-fig. 9 shows the corresponding range in 2V. Text- 
fig. 10 emphasizes the similarity between lava and dyke phenocrystal pyroxenes as 
regards their 2V values. 



244 



PETROLOGY OF LORD HOWE ISLAND, PART 1 



2V(y) 


49-2° 


y 


1-734 


y^C 


45r 



Table IV 

Mean optical values for Pyroxenes 

Groundmass Phenocrystal 
pyroxene pyroxene 

52-8° 
1-720 
44i° 

Since the optical properties show that all pyroxene phenocrysts (from both lavas 
and dykes) have virtually identical compositions, and that the same is true for the 
groundmass pyroxenes, analysis of a pyroxene from each generation should suffice 
to represent the composition of all pyroxenes from that generation in both lavas 
and dykes. A dyke (BM.1966, P5, 66) from North beach was used for separation 
and analysis of groundmass pyroxenes, while an oceanite-ankaramite (BM.1966, P5, 
33) from a height of 720 ft in the Gower-Lidgbird sequence was taken for separation 
and analysis of phenocrystal pyroxene. 



c 
o 



o 
> 



o 



o 

z 



-6 
5 

4 

^3 

2 
1 



(aj phenocrysts 



(b) groundmass 



1-717 1-719 1-721 1-723 1-725 1-727 1-729 1-731 1-733 W35 1-737 1-739 

Refractive Index (8) 

Fig. 8. Range in refractive index (y) values for (a) phenocrystal pyroxenes and (b) ground- 
mass pyroxenes (Gower-Lidgbird lavas and dyke rocks combined). 



The analyses of these two pyroxenes are shown in Table V together vdth the values 
of y, 2V, and of the cell sides and the angle j8. In the triangular diagram (Text-fig. 
11) the composition, derived from analysis, is compared with the composition de- 
rived from X-ray measurements of cell sides and of /S. The augite phenocrysts from 
Lord Howe Island, though slightly richer in magnesium and poorer in calcium, 
match, closely, those from a picrite basalt from Gough Island (LeMaitre, 1962). 
Moreover the trend in the compositional range between phenocrysts and groundmass 
is very similar to the initial trends plotted for alkali basalt pyroxenes in general, 
i.e. it is parallel to the Di-Hd join, the molecular calcium percentage remaining con- 
stant with increase in iron. In tholeiitic pyroxenes, by contrast, the initial trend 



PETROLOGY OF LORD HOWE ISLAND, PART I 



245 



8 



6 



(A) 



^p^^^^^^ 



47 4 8 49 50 



46 



en 

c 
o 



03 

> 

Q) 
Vi 
£1 
O 



•12 



10 



8 



■6 



51 52 53 



2Vy 



(B) 



FWW^Wq 



49 



I 

50 



51 5 2 5 3 54 5 5 



deg rees 



Fig. 9. Range in optic axial angles in (a) groundmass and (b) phenocrystal pyroxenes. 



246 



PETROLOGY OF LORD HOWE ISLAND, PART I 




degrees 

Fig. 10. Range in optic axial angles in pyroxene phenocrysts in (a) The Gower-Lidgbird 

lava series and (b) The dyke rocks. 



is one of impoverishment of calcium as well as of enrichment in iron (LeMaitre, op. 
cit.). 

The slight deviations, in the same sense, between the Ca : Mg : Fe ratios deter- 
mined by chemical analysis on the one hand and by X-ray analysis on the other 
may be due to the relatively large amount of aluminium in these pyroxenes, and 
especially to the Al in 6-fold co-ordination. Although the Al allotted on the formal 



Mg 



PETROLOGY OF LORD HOWE ISLAND, PART I 

CaMg CaFe 



247 




1 O Pyroxene phenocrysts ; Composition from x-ray data 

II II , . 

1 A " " analysis 

- data 



groundmass 



X- ray 



2 O 

M II II H I • 

2 A analysis 

Fig. II. Pyroxenes ; comparison of chemical and X-ray data. 



6-oxygens basis of the analyses is 0-047 ^^^ ^^e phenocryst and 0-036 for the ground- 
mass there is, of course, no method for apportioning the total Al between the Al^ 
and AI4 sites. According to Brown (i960) the critical value of Alg which the pyrox- 
enes can accommodate without alteration of cell is 0-05. A surplus causes a reduc- 
tion in the value of b. As such a reduction can be observed to have occurred in 
both analysed pyroxenes from Lord Howe Island it seems reasonable to assume that 
more Al occurs in 6-fold co-ordination than the amounts derived formally from the 
analyses — in which case some Ti might possibly go into the Z position to preserve 
the necessary balance. 

The relations between the compositions derived from the optical parameters y 
and 2V and those obtained from the analyses are shown in Text-fig. 12. This shows 
that the compositions derived from the optics are at considerable variance with the 
analytical compositions. The discrepancy is almost entirely due to the refractive 
index values, which are as much as -015 to -020 too high, when interpreted on Hess' 
(1949) diagram. This diagram was used because Hess plotted the y index which was 
the index measured on the Lord Howe Island pyroxenes. If the most recent diagram 
(Brown and Vincent, 1963) is used and if y-)3 is taken as -02 then the discrepancy is 
slightly reduced, but is still just over -01 for the phenocrystal pyroxene (in which the 
refractive index may be assumed to have been accurately measured). It should 



248 



PETROLOGY OF LORD HOWE ISLAND, PART I 



Table V 
Analyses of Pyroxenes (Analyst : A. A. Moss) 





Pyroxene 


Pyroxene 


Number of ions on 


the basis of 6 


oxygens 




phenocryst 


groundmass 






















BM.1966, P5, 33 


BM.1966, P5, 66 


BM 


1966, P5, 33 


BM.1966, P5, 66 


SiOz 
TiOa 


50-3 

I-O 


49-3 
1-6 


Si 
Al 


1-844 ^ 
0-156 J 


> 2-00 


1-838 "1 

:: > 2-00 
0-162 


AI2O3 


47 


4-5 


Al 


0-047 




0-036 * 




FezOa 


0-9 


2-5 


Ti 


0-028 




0-045 




FeO 


5-4 


7-6 


Fe3 + 


0-025 




0-070 




MnO 


O-I 


o-i 


Fe2 + 


0-165 




0-237 




MgO 


lyi 


14-0 


Mn 


0-003 


► 2-03 


0-003 


;. 2-01 


CaO 


20-4 


19-6 


Mg 


0-934 




0-778 




NazO 


0-33 


0-76 


Ca 


0-801 




0-783 




K2O 


0-05 


O-II 


Na 
K 


0-023 

0-002 




0-055 

0-005 





Total 



100-28 



100-07 



Optical data 
BM.1966, P5, 33 BM.1966, P5, 66 



2Vy 
7 



52-7° 
1-722 



49-2° 
1-733 



X-ray data 
BM.1966, P5, 33 BM.1966, P5, 66 



a 
b 
c 



9-747 A 
8-909 A 
5-266 A 
106-334° 



9-757 A 
8-917 A 

5-274 ^ 
106-331° 



be noted that Deer and Abbott (1965) obtained the same orders of difference and 
in the same sense, between the expected and the measured refractive indices for 
clinopyroxenes from gabbro cumulates in east Greenland, for which the total Al and 
the Ti percentages are similar to those of the Lord Howe Island pyroxenes. As these 
authors remark " data bearing on the relative effects of tetrahedrally and octa- 
hedrally co-ordinated Al on the refractive index of clinopyroxenes is both scanty 
and conflicting ". They consider " the b and a sin j8 parameters are superior to the j8 
index and 2V as a guide to composition ", with which opinion the author is in com- 
plete agreement. 

Zoning of pyroxenes. This occurs in both phenocrysts (PI. 12, 3) and ground- 
mass although it is never intense. It is more common in the dykes than in the flows. 
In the dykes determinations of 2V on the cores and on the exteriors of pyroxenes 
showed that 2V is, on the average, 3-0° greater for the cores of phenocrysts than it is 
for their margins (mean values are 527° and 49-7° respectively). For the ground- 
mass 2V is 3-5° greater at the centre than it is at the exteriors (50-4° against 46-9°). 
A difference of 3° to 31° in 2V suggests that the calcium molecular percentage is 
about 3% lower at the margins. Having regard to the fact that the optical-chemical 
relationships for monocUnic pyroxenes are not well estabUshed, a microprobe analysis 
was made on a zoned crystal from a dyke (BM.1966, P5, 96) from Rabbit Island. 



PETROLOGY OF LORD HOWE ISLAND, PART I 



249 



0) 




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250 



PETROLOGY OF LORD HOWE ISLAND, PART I 



Approximately 25 measurements were made on the margin and on the centre. The 
results, in weight percentages of elements (Fe includes ferrous plus ferric iron) were : 

Mg 
Centre : Fe 6-6o Ca 15-17 Si 22-62 Mg 9-61 : — = 1-46 



Margin : Fe 7-02 Ca 15-29 Si 22-14 Mg 8-50 



Mg 
Fe 



= I-2I 



It is significant that these analyses show no appreciable change in calcium ; at the 
periphery however iron is higher and magnesium is lower. 



CaO 



(CaMg) O 




MgO 



pyroxene 



• rock 



^FeO 



□ pyroxene 
Lord Howe 
Island 



Orock 
Lord Howe 
Island 



Fig. 13. Pyroxenes of olivine-basalt magmas in relation to the rocks in which they occur ; 
after W. Q. Kennedy (1933). i. Satakunta, Finland. 2. Magdesprung, Hartz. 3. 
Mount Taylor region, New Mexico. 4. Azores. 5. Etna. 6. Hawaii. 7. Lord Howe 
Island. 



PETROLOGY OF LORD HOWE ISLAND, PART I 251 

Text-fig. 13 shows that the pyroxene which separates from the Lord Howe Island 
magma is richer in lime than the magma itself. In this it accords with pyroxenes 
from olivine-basalt magmas in general, six of which are reproduced from Kennedy 
(1933)- 

{d) Olivines 

Olivine occurs as phenocrysts in all the porphyritic lavas of the Gower-Lidgbird 
series. It is completely unaltered in two flows, although in a further two the altera- 
tion is very slight. In the great majority of the remaining lavas of this sequence 
phenocrystal olivine is iddingsitized to greater or less extent. In most instances only 
the periphery is affected (PL 11, 3) but in a few instances almost the whole crystal 
has been iddingsitized. In three flows from the middle of the sequence (BM.ig66, 
P5, 24, 26 and 27) and in one from the base (BM.1966, P5, 40) olivine phenocrysts 
are serpentinized. In one of these flows (BM.1966, P5, 26) the alteration is very 
slight. 

In the groundmass of the Gower-Lidgbird lavas olivine occurs in most of the 
flows, but it is more abundant in the lower part of the sequence (300 to 1500 ft) 
than it is in the upper part (1500-2800 ft) . 

Olivine phenocrysts also occur in some of the dykes, but they are much less plenti- 
ful than in the flows. As previously stated olivine only occurs significantly in 8 
dykes out of the 52 examined. (In a further 2 dykes it is very sparse.) Five of 
these 8 dykes are rich in olivine phenocrysts. They include two dykes from the 
Admiralty Isles, and one each from Middle beach, Ned's beach, and the quarry by the 
lagoon. Olivine occurs sparingly in the groundmass in only six dyke rocks, all of 
which are among those carrying olivine phenocrysts. In the dykes the olivine is 
always serpentinized (never iddingsitized) to a greater or less degree (PI. 11, 2). 
In some rocks, e.g. BM.1966, P5, 78 from Ned's beach and BM.1966, P5, 102 from 
the largest Admiralty Isle, the crystals are only very slightly altered along cracks. 
In others, e.g. BM.1966, P5, 71, from North beach and BM.1966, P5, 99 from the 
quarry near the lagoon, serpentine sometimes completely pseudomorphs original 
olivine. 

Composition. The compositions of the olivines were derived from the refractive 
index values determined by the single variation method. Text-fig. 14 shows the 
composition of the olivines from the Gower-Lidgbird lavas and from the dykes. 
In the lavas the composition of the olivine phenocrysts shows no significant variation 
in the flows of the upper 1300 ft, where it averages 26% (molecular) of Fe2Si04. 
In the lower 1500 ft (i.e. in those flows which also carry more olivine in the ground- 
mass) the composition of the phenocrystal olivine shows minor, albeit abrupt, 
variations from 18% to nearly 28% Fe2Si04. The average composition is 23% 
in this lower part of the sequence. As the diagram shows there is a slight but 
definite trend towards enrichment in iron from the base of the series to a point about 
half way up. Groundmass olivine could only be accurately determined from two 
flows (at about 1500 ft) from which a mean value of 36% Fe2Si04 was obtained. 
Since the phenocrysts of these particular flows have a composition of 21% Fe2Si04, 
the groundmass olivine has 15% more Fe2Si04 than the phenocrysts. A third flow 



252 



PETROLOGY OF LORD HOWE ISLAND, PART I 
COMPOSITION OF OLIVINES 



LOCALITY 
Quarry lagoon - 

Ned'b Beech 
Middle Bee ch 
Admiralty islets 

North Beach 



Height 
a bove sea 
level 

854 2800 

Meters Ft 



610 

m 



2000 

Ft 



305 

m 



1000 

Ft 



Mol % Fe,SiO. 

i 4 



14 



18 



2 2 



26 





J 1 1 1 1 1 1 1 

Dykes 




- 


O 






- 


o 


O Phenocrystol 


Olivine 


- 


oo 


• Groundmass 


Ol ivine 


- 


00 






- 


GOO 








Gower 


L idgbi rd lava s 




- 


Q 






. 


jo 








0| 














. 


Ol ^ 








1 ° 






ol 








o 1 


O 






O / 


O 


• 












1 


• 






1 








O ; 


• 






°^ 


Oq 














/ 

O/ 
O / 






■ 


/ 
/ o 

o / °o 






1 


1 1 1 1 1 1 1 


1 1 1 1 


— 1 1 



14 



18 



22 



26 



30 



34 



Mol % FeSiO 

2 4 



Fig. 14. Composition of olivines from the Gower-Lidgbird series and from the dykes. 



PETROLOGY OF LORD HOWE ISLAND, PART I 253 

(1370 ft) with very small phenocrysts has olivine of an intermediate composition 
between groundmass and phenocrysts. 

In those dykes with phenocrystal oKvine, the composition of the latter ranges 
from 17^% to 24% Fe2Si04. Iron is lowest in the olivines of the dykes from North 
beach, four of which carry olivine phenocrysts, with a mean composition of 18% 
Fe2Si04. Olivine phenocrysts from all other ohvine bearing dykes have a mean 
composition of 22|% Fe2Si04. This difference in composition (4^% Fe2Si04) is 
outside the hmits of experimental error and must be regarded as significant. With 
the exception of the North beach dykes, the mean composition of the olivine pheno- 
crysts of the dyke rocks is thus virtually identical with the mean composition of 
phenocrystal oUvines from the lower part of the Gower-Lidgbird lava sequence. 

{e) Iron Ores 

The ores, which are of universal occurrence in both lavas and dykes were examined 
by means of the microprobe. Total Fe and Ti weight percentages were obtained. 
The results for the lavas and dykes are plotted in Text-fig. 15. The ores fall into 
two groups ; the first is close to theoretical ilmenite, while the other appears to 
approximate to a Ti-poor Fe-rich ulvospinel. 

The points show considerable scatter and much of this is undoubtedly due to the 
practical difficulties of resolution with the probe on such small particles. There is 



35. 


TiO, 


' 




30- 


TiO- >^^ 

w \ Ay ilmenite 

c. of G. qoSS^ n ^^ ^ 

lavas Q ^\^ \ 


of G.for dykes 
Ore 


from Gower- Lidgbird lavas 




^ ^ A Ore 


from the dyke rocks 


25 ■ 


\ 










X \ 


Y ulvospinel 


20 ■ 




\ 


\ 






\ 


C. of G. for dykes 

On. 






\ 


^° y V 


15 


C. of G. for lavas 


A° I? Ov Xj=eO 









10 






^ Fe^O, 



30 40 ' SO 60 

Wf percent Fe"& Fe'" 

Fig. 15. Iron-titanium ratio for lavas of the Gower-Lidgbird sequence and for the dyke 

rocks. 



254 



PETROLOGY OF LORD HOWE ISLAND, PART I 



almost certainly a systematic error (in addition to the random scatter), for a consid- 
erable number of points plot outside the Ti02-Fe203 join in the Ti02-Fe203-FeO 
triangle. In these Fe and Ti are deficient and there is too high a proportion of oxy- 
gen. This may possibly be due to the traverse by the beam of silicates in addition 
to the ores. 



U- 

+ 

5(U 



c 
O 



0) 

O) 

D 

c 

0) 

u 

k. 

0) 
Q. 



60 



50 



40 _ 



magnetite 



ulvospinel 



—J— 
10 



— r- 

20 



Wt percentage of Titanium 



Fig. i6. Iron-Titanium ratios for individual ore grains in flow BM.1966, P5, 28 at iioo ft 

above sea level. 



PETROLOGY OF LORD HOWE ISLAND, PART I 



255 



HEIGHT 

Feet meters 

2700 823-5 



2450 



2350 



2150 

2 COO 

1920 

1800 

1500 

1490 

1475 

1455 

1100 
1080 
1060 

650 
570 

500 
330 



747-2 



716-7 



855-7 
610-0 
575-5 
549-0 
457-5 
454-4 
449-8 
432 7 

335-5 
329-4 
323-3 
198-2 
173-8 
152-5 

100-6 



10 30 50 70 90 

100 80 60 40 20 



100 ulvospineOv' 
ilmenite j * 



Fig. 17. Relation between height of flow above sea level and type of ore, Gower-Lidgbird 

series. 



256 PETROLOGY OF LORD HOWE ISLAND, PART I 

In spite of the scatter and probable " translations " approximately perpendicular 
to the ilmenite-ulvospinel join, of the centres of gravity of the groups it seems 
reasonable to postulate a small difference in composition between the ilmenites of 
the dykes and those of the lavas. The former have on average 4% more Fe and 1% 
less Ti than do the latter. The dyke ilmenites have begun to move along the il- 
menite-haematite solid solution join with cooUng and oxidation, but are still close to 
theoretical ilmenite. The ilmenites of the Gower-Lidgbird lavas do not show this 
trend, but appear to be more affected by the systematic error mentioned above and 
their centre of gravity is near the Ti02-Fe203 join. Nevertheless both their Fe 
and Ti percentages are still within 3% of those of ilmenite. 

In the Ti-poor ulvospinel group no significant difference between the ores of the 
lavas and of the dykes can be detected. It is probable that the first ore to form was 
an ulvospinel of the ulvospinel-magnetite solid solution ; with cooling and oxidation 
this early crystallized ore would move towards the magnetite end of this soHd solu- 
tion series, possibly with exsolution of lamellae of ilmenite. That this does occur in 
some instances can be seen by reference to Text-fig. 16 in which the Fe and Ti 
percentages of individual grains from a fine-grained lava (BM.1966, P5, 28, iioo ft 
a.s.l.) have been plotted. The trend of the resulting points is close to that of the 
magnetite-ulvospinel join. It is also interesting to note with reference to this 
diagram that the Mg percentage, which varies from 0-4% to 2-0% is directly propor- 
tional to the percentage of Ti. 

In the Gower-Lidgbird series there is a definite trend from ilmenite bearing flows 
in the lower layers to ulvospinel bearing lavas near the top. The plot (Text-fig. 17) 
shows that the lowest flows (with one exception) have only ilmenite, while the upper 
flows (again with one exception) have only ulvospinel. The lavas near the middle 
of the series contain both types of ore (again with one exception, BM.1966, P5, 16 
from 1800 ft), which has only ilmenite. The dyke rocks contain both ilmenite and 
ulvospinel but the proportion varies greatly. Three dykes at North beach con- 
tain an average of 34% ulvospinel, in contrast with an average value of 83% 
ulvospinel for three dykes from other locaUties. But more determinations would 
be needed to test the occurrence of any relation between locality and type of ore. 

IV. BULK CHEMICAL ANALYSES 

The results of fourteen chemical analyses (9 flows and 5 dyke rocks) are given in 
Table VI, and Table VII shows the molecular norms. In the absence of sufficient 
felsic variation it is not possible to construct an alkali-lime index diagram, but the 
triangular diagram for magnesia, iron oxides and alkalis is shown in Text-fig. 18. 
The K2O, Na20, CaO triangular diagram is also shown in this figure. Text-fig. 19 
shows the relation between AI2O3 and total alkaUs. Here the Lord Howe Island 
rocks are compared with basaltic types from the Circum- Japan sea province (see 
Kuno 1959) and with Nockolds' (1954) " normal alkali basalts " and " average 
alkali andesites ". This shows that nearly all the Lord Howe Island basalts faU 
within Kuno's alkali basalt group. In spite of the small corundum content in the 
norm of two Lord Howe Island basalts, none approaches Kuno's high alumina basalt 
group. The figure also demonstrates a close conformity between the Lord Howe 



PETROLOGY OF LORD HOWE ISLAND, PART I 



257 













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gbird, 300 ft, 
gbird, 270 ft, 
ch, BM.1966, 
ary carbonati 

:h, BM.1966, 
miralty Islet: 
ddle beach, E 




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ine basalt, Mt Gower, 2 
olivine basalt, Mt Gow 
ite, Mt Gower, 2200 ft, 
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andesite, Mt Lidgbird, 
olivine basalt, Mt Lidg 
oUvine basalt, Mt Lidg 




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258 



PETROLOGY OF LORD HOWE ISLAND, PART I 



u 

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PETROLOGY OF LORD HOWE ISLAND, PART I 



259 




O 
o 
U 





TS 




c 




iS 








"in 




I— I 


\° 


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% 


w 




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26o 



PETROLOGY OF LORD HOWE ISLAND, PART I 



15 - 



BM1966,P5.89 • 

• • • 




. •' X 

•a t. 


• 
• 


2 A \\ • 
83 26 


• 

• 


• 
a8 
_ 1 — 


Si ©2 45-4 
1 



20 



15 



2 3 

Wt% 



o 

/I • W SlO- 47 5-50 
° 102 A 82 V22 2 

27 • 



20 



15 



.14 



O D 






64 A /• • 






D 
D 



A 
80 



• SiO^ 50-52-5 



10 



Hap + KgOWt % 



X Normal Alkali basalts (Nockolds) 
T Average Alkali andesite (Nockolds) 
• Alkali basalt 



*~*High alumina basalt 
° tholeiite 



Circum-Japan sea 



provrnce 



^ Lord Howe Island 

Fig. 19. Alumina — total alkali relationships for the Lord Howe Island basalts for varying 
Si02 percentages, compared with those of basalts from the circum-Japan sea province : 
after H. Kuno. 



PETROLOGY OF LORD HOWE ISLAND, PART I 261 

Island basalts and Nockolds' " average alkali andesite ". With regard to the Lord 
Howe analyses three points call for special mention ; first, in grouping the basaltic 
lavas for comparative purposes BM.1966, P5, 14 has been omitted because its field 
relations suggest that it is an inclined transgressive sheet (late differentiate) and not 
a flow. Second, the 2-2% of normative nepheline in the highest flow {BM.1966, P5, 
I norm la) is due to the albitization of the plagioclase in this flow (which is otherwise 
unaltered). Norm ib for this rock shows the result of reallocation of the CaO and 
Na20 in the mean proportions in which they occur in the Lord Howe Island lavas as a 
whole. This results in a very slightly oversaturated norm (|% quartz) instead of an 
undersaturated one. Third, the only lava to show normative quartz (BM.1966, 
P5, 37 norm "a") owes the 3% excess Si02 almost entirely to the unusually high 
state of oxidation of the iron. Re-apportionment of the total iron in this rock to 
accord with the mean FeO : Fe203 ratio for the remaining lavas reduces the 3% 
excess Si02 to a mere |% (norm " b "). The remaining lavas contain normative 
olivine, but only two, one of which is the transgressive sheet (BM.1966, P5, 14) al- 
ready mentioned, show small amounts of normative nepheline. The unusually 
high H2O-I- content of both flows and dykes (mean H20-f = 2-0%) is probably due 
to the relative abundance of the zeolite chabazite as a late stage product (often 
infilling vesicles) . 

In order to determine whether there is any trend in the lavas in silica saturation 
or undersaturation the norms were recalculated after conversion of all Fe203 to 
FeO and Table VIII shows the resulting deficiency in Si02 after forming hypersthene, 
but before using excess hypersthene to form olivine. 

Table VIII 

GOWER-LIDGBIRD LaVAS ; RELATION BETWEEN 
HEIGHT OF FLOW AND SILICA DEFICIENCY 



BM. No. 


Height (ft) 


SiOa deficiency 


1966, P5 


2600 




2 


-9-7 


8 


2200 


— 14-6 


24 


1455 


— 100 


26 


1210 


-9-2 


27 


1120 


-7-6 


37 


300 


-3-7 


40 


270 


-4-8 



BM.1966, P5, I has been omitted because of the albitization of the plagioclase, 
and BM.1966, P5, 14 has been left out for the reasons previously given. The table 
shows a general tendency towards increased Si02 undersaturation with height, 
although this trend is reversed in the highest flow. The trend towards undersatura- 
tion accords with the trend towards soda enrichment of the plagioclase with height 
that has already been described (Text-fig. 3). 



262 PETROLOGY OF LORD HOWE ISLAND, PART I 

Comparisons. Table IX shows the average composition of 8 lavas and 5 dykes 
(combined) from Lord Howe Island and gives the compositions of those lavas 



Table IX 
Comparison of average Lord Howe Island Basalt with similar types 

FROM other localities 

(100% water-free basis) 



Si02 


48-67 


48-45 


48-0 


50-6 


48-45 


48-45 


TiOa 


2-63 


2-2 


2-85 


1-6 


4-3 


2-2 


AI2O3 


14-97 


15-4 


14-7 


14-5 


13-3 


16-65 


FeaOs 


3-99 


3-9 


4-0 


4-3 


4-05 


3-35 


FeO 


7-67 


8-05 


7.9 


7-1 


8-3 


7-3 


MnO 


0-16 


0-15 


0-2 


0-2 


0-15 


0-2 


CaO 


8-66 


9-5 


9-55 


8-6 


77 


9-5 


MgO 


7-87 


7-25 


7-3 


8-4 


8-2 


6-05 


NaaO 


3-33 


3-3 


3-8 


2-9 


3-45 


3-5 


K2O 


1-34 


1-2 


1-2 


1-3 


1-55 


2-1 


P2O5 


0-46 


0-6 


0-5 


0-4 


0-55 


0-7 


CO2 


0-23 


— 


— 


O-I 


— 


— 


CraOa 


0-02 




NORMS 






' 


cal 


0-6 


— 


— 


o-i 


— 


— 


ap 


0-95 


i'3 


i-i 


0-8 


1-2 


1-45 


il 


3-7 


3-0 


4-0 


2-3 


6-0 


3-1 


or 


8-0 


7-0 


7-0 


7-5 


9-25 


12-5 


ab 


2975 


2975 


2975 


25-5 


31-25 


24-0 


an 


22-0 


2375 


19-5 


23-4 


16-25 


23-15 


mt 


4-2 


4-05 


4-2 


4-5 


4-3 


3-45 


di 


13-2 


15-8 


19-6 


13-2 


14-8 


17-2 


hy 


7-7 


3-2 


— 


22-7 


8-4 


— 


ol 


9-9 


12-15 


I2'I5 


— 


8-55 


IO-8 


ne 


— 


— 


2-7 


— 


— 


4-35 



1 Lord Howe Island. Mean of 8 lavas and 5 dykes 

2 Olivine basalts, East Otago, New Zealand : Benson (1946) 

3 " Average alkali andesites " : Nockolds (1954) 

4 Alkali basalts. Central Victoria, Australia : Edwards (1938) 

5 Olivine basalts, Tutuila, Samoa : Daly (1927) 

6 Basalt-trachybasalts, circum Japan Sea : Kuno (1959) 

Fig. 20. Sample distribution (weight %) of TiOa, Si02, AI2O3, FeO, and of S Salic and the 
Thornton-Tuttle index in oceanic Tertiary volcanics ; after Felix Chayes ig64. (Oceanic, 
solid line and open circles ; circum-oceanic, dashed line and solid circles.) The thick 
vertical solid line (LHI) shows the mean percentages of the named oxide and of E salic 
and the Thornton-Tuttle index for 15 analysed rocks from Lord Howe Island. 



PETROLOGY OF LORD HOWE ISLAND, PART I 



263 




0-7 1*4 2-1 2-8 




TiO, 



SiO, 





LHI 


100 




i 




P 


R 


50- 


I 1 


' I 




1 Q 


k 



LHI 



3 6 9 120 

FeO 



LHI 




13 16 19 



AI2O3 



160- 



120 



80 



40 



y ^ 



40 50 60 70 



§ Sal/c 



LHI 




20 30 40 50 6'0 7j 

Thornton -Tuttle Index 



Weight percentage of oxide 



264 



PETROLOGY OF LORD HOWE ISLAND, PART I 



that most closely match the Lord Howe average. It will be seen that the best 
matches are to be found among the New Zealand basalts (Benson, 1946) and in the 
" average alkali andesites " of Nockolds (1954) (probably mainly from the Hawaiian 
group). The alkali basalts from Victoria described by Edwards (1938) are also simi- 
lar, but Si02 is a little higher and Ti02 considerably lower. The Samoan oUvine 
basalts, by contrast (Daly, 1927), have distinctly higher Ti02 and i|% less AI2O3, 
but otherwise are fairly close to the Lord Howe Island average. In the Japan Sea 
province, K2O is distinctly higher than in the Lord Howe Island group, but otherwise 
the match is good. 

It is also instructive to compare the mean percentage of each oxide of the analysed 
rocks from Lord Howe Island with the distribution of this oxide weight per cent 
as given by Chayes (1964) for a large number of oceanic and circum-oceanic Tertiary 
volcanics. Text-fig. 20 shows Chayes' sample distributions. Although in all six 
distributions, the Lord Howe Island mean values fall close to the peak for the oceanic 
rocks, they are, in each instance, sUghtly displaced towards the circum-oceanic peak. 
The displacement is small, but the fact that it occurs in every instance is significant. 

Close matches can also be found for the individual Lord Howe volcanics. Table X. 



Table X 
Matches for individual Lord Howe Island rocks 



SiOz 


49-6 


50-0 


50-3 


49-8 


47-9 


47-7 


44-5 


43-32 


TiOz 


2-3 


1-8 


2-4 


2-0 


2-5 


3-2 


1-8 


2 -02 


AI2O3 


i8-i 


18-4 


17-2 


16-3 


i6-o 


15-2 


IO-6 


9-II 


FeaOs 


2-9 


4-2 


3-8 


2-9 


2-2 


2-3 


5-1 


4-94 


FeO 


8.4 


6-8 


8-1 


8-6 


9-3 


8-7 


8-7 


8-42 


MnO 


0-13 


0-05 


O-I 


0-3 


— 


— 


0-14 


— 


MgO 


3-9 


3-3 


5-2 


5-4 


9-8 


9-7 


17-0 


17-02 


CaO 


5-4 


67 


4-7 


8-2 


8-6 


8-9 


7-5 


9-28 


NaaO 


5-2 


4-8 


5-2 


3-2 


2-6 


2-7 


1-9 


1-86 


K2O 


1-8 


2-0 


2-2 


2-1 


i-i 


1-6 


0-64 


0-86 


H2O + 


1-4 


0-9 








— 


— 


2-0 


2-00 


H2O- 


O-I 


0-2 








— 


— 


O-I 


1-16 


P2O5 


0-39 


0-34 


0-8 


0-4 


— 


— 


0-23 


0-34 


CO 2 


— 


— 


■ — 


0-8 


— 


— 


0-2 


— 



Totals 99-62 99-49 loo-o loo-o loo-o loo-o 100-41 100-33 



Mt. Gower, 1920 ft. 



1 Lord Howe Island (BM. 1966, P5, 14) ; trachybasalt ; 

2 Trachydolerite : St Helena ; Daly (1927) 

3 Lord Howe Island (BM. 1966, P5, i) ; andesine basalt 

4 Andesine basalt : Victoria; Edwards (1938) 

5 Lord Howe Island (BM.ig66, P5, 89) ; olivine basalt 

6 " Basalt average " : Gough Island ; LeMaitre (1962) 

7 Lord Howe Island (BM.i966,P5, 8) ; oceanite ; Mt. Gower, 2200 ft. 

8 Ankaramite : Mauritius; WalkerandNicolaysen (1953) 



Mt. Gower, 2700 ft. 
dyke, Middle Beach 



PETROLOGY OF LORD HOWE ISLAND, PART I 265 

Thus BM.1966, P5, 14 the latest differentiate from Lord Howe Island, finds a close 
counterpart in a trachydolerite from St Helena, described by Daly (1927). The 
albitized lava (BM.1966, P5, i) from the top of the Gower-Lidgbird sequence has an 
analysis which is close to the mean of certain andesine basalts from Victoria de- 
scribed by Edwards (1938). The albitization of the feldspar in the Lord Howe rock 
is shown in the table by the " reversal " of the CaO and Na20 percentages, compared 
with those from the Victorian basalts. 

Matches can also be found with basalts from other Atlantic islands. The analysis 
of an olivine basalt from a dyke at Middle beach, Lord Howe Island (BM.1966, P5, 
89) can be compared with the " basalt average " as given by LeMaitre (1962) for 
Gough Island, in the South Atlantic. 

The oceanites and oceanite-ankaramites of Lord Howe Island, typified by the 
analysed example BM.1966, P5, 8 from 2200 feet above sea level are matched by 
similar types from Mauritius (Walker and Nicolayson 1953). 

It is likely that these Lord Howe Island oceanites (of which five examples were 
collected in 1700 ft of the Gower-Lidgbird sequence) were formed by the accumula- 
tion in basaltic magma of olivine (and much smaller amounts of pyroxene) pheno- 
crysts. The oceanite contains 32% of olivine of composition Fo74Fa26. If this be 
subtracted from the bulk composition of the rock the remaining groundmass has a 
composition, as well as a norm, similar to some dyke rocks e.g. BM.1966, P5, 64 
from North beach. There is consequently no necessity to postulate assimilation of 
CaO or any other material. 

V. RELATION BETWEEN COMPOSITION AND DISTANCE 
FROM RIDGE CREST 

The Niggli quartz number for the average Lord Howe Island basalt is —25-6 
and that for the trachyandesite (the most differentiated member) is —34.5. For 
basalts and their most differentiated products of islands near the East Pacific Rise 
or the Mid-Atlantic Ridge, McBirney and Gass (1967) have found a relation between 
the Niggli quartz number and the distance of the island from the crest of the ridge. 
In the eastern Pacific the curve for the basalts is almost flat over a distance of 5000 
km. to the west of the rise and 2000 km. to the east. The mean value over this range 
is —25, virtually identical with that for Lord Howe Island. In the eastern Pacific, 
the latest differentiates have Niggli quartz numbers which for the most part are well 
above those for their parent magmas, whereas the reverse is true for Lord Howe 
Island and for the majority of the Atlantic islands. In the Atlantic ocean the curve 
for the Niggli quartz numbers of the islands to the west of the Mid-Atlantic Ridge 
shows a definite inclination. Here a value of —25*6 (the 'average' Lord Howe 
basalt) indicates a distance from the ridge of 270 km., compared with the actual 
distance of Lord Howe Island from the crest of its Rise of 340 km. In the same region 
the value of —34-5 (the most differentiated Lord Howe type) would correspond 
with a distance from the Ridge of 510 km. Thus the Lord Howe Niggli quartz 
numbers are of the same order as those of rocks that lie similar distances to the west 
of the Mid- Atlantic Ridge. 

It should be noted that the Lord Howe Rise is now aseismic (Gutenberg and Rich- 



266 PETROLOGY OF LORD HOWE ISLAND, PART I 

ter, 1954) and in this respect it shows a marked contrast to the Mid- Atlantic Ridge 
and the East Pacific Rise, which are both more youthful structures with active vul- 
canicity. As Gutenberg and Richter remark, " Seismicity must have changed 
greatly in the course of geologic time — a few tens of thousands of years is ample time 
for extensive and significant changes in the local distribution of stress". Con- 
sequently the Lord Howe Rise may have been a seismic region up to the mid- 
Pliocene when vulcanicity ceased. 

Van der Linden (1967) considers that the core of the rise was formed from an early 
Palaeozoic geosyncline. He concludes from the low angle unconformity on the north- 
east flank of the rise (discovered during a 1966 seismic survey by the " R.V. Eltanin ") 
that the crest of the rise was above sea level in the late Palaeozoic and early Meso- 
zoic, and that it was the source of material for the New Zealand Permian and early 
Mesozoic sediments, which according to Fleming (1962) are derived from a landmass 
to the west. 

Apart from the rather minor exception of the Ulladulla Trench which lies about 
100 miles from the south eastern AustraHan coast, and is about 700 miles in length 
and about 17000 ft in depth, there is no trench off the east AustraUan coast, but a 
vast region 2000 miles wide between east Australia and the Tonga Trench which Chayes 
(1965) includes in his " shallow sea " or " Mediterranean " category. Lord Howe 
and Norfolk Islands are the only islands in this region, and exemplify Chayes' con- 
cept that " where the continent-ocean border is not marked by a Cenozoic trench, 
Cenozoic volcanics are not likely to be abundant and those which do occur are Ukely 
to be petrographically oceanic ". Although, as shown, Lord Howe Island is built of 
basalt almost typical of Chayes' petrographically " oceanic types ", it has as stated 
a tendency towards his petrographically " circumoceanic " class. It may be noted 
that the alkaline eastern Otago suite of South Island, New Zealand, which the Lord 
Howe basalts closely match, provides one of the main exceptions to Chayes' con- 
ception, since it is geographically circumoceanic, but petrographically oceanic. 
The close match between the Lord Howe Island basalts and those from south eastern 
AustraUa calls for comment, because the latter are clearly circumoceanic. However 
Macdonald (1949), confirming the findings of Edwards (1935), has pointed out that the 
olivine basalt-trachyte associations, typical of the Pacific province (and other ocean 
basins) are not wholly restricted to these oceanic areas, but are found, additionally, 
in some continental settings, but only in areas that have experienced no major 
orogeny for very long periods previous to the eruptions, whereas the calc-alkaline 
magmas typically accompany and follow orogeny. The south eastern Australian 
region has not been subject to any orogenic epoch since the late Devonian and is a 
region of Mesozoic and Tertiary stability. 

The remaining matches that have been noted (Table X) for the Lord Howe Island 
rocks are with truly oceanic basalts (Nockold's " alkali andesites " are mainly from 
Hawaii) and therefore call for no special comment. 

Whereas Lord Howe Island lies weU to the west of the crest of the Lord Howe 
Rise, Norfolk Island lies on the crest of the Norfolk Island Ridge. Analyses of the 
Norfolk Island volcanics would therefore be particularly desirable, in order to com- 
pare the degree of silica saturation of the parental rock and its most differentiated 



PETROLOGY OF LORD HOWE ISLAND, PART I 267 

member with the values obtained for Lord Howe Island. McBirney and Gass 
(1967) have shown a clearly increased silica saturation from basaltic and differenti- 
ated rocks on and near the crests of the East Pacific Rise and the Mid-Atlantic 
Ridge. An extension of their methods to the south west Pacific should help to show 
whether their findings are of more general application. 

VI. NOTES ON THE OLDER SERIES 

Inclined altered series The older series of tilted lavas and dykes (PL 15,2) are not 
a pleasing study owing to their alteration, mainly to carbonates and chlorite with less 
common conversion of the pyroxene to calcite. Olivine, where present, is completely 
altered to a talc-ore residuum. In those flows in which plagioclase and pyroxene 
are sufficiently fresh to allow optical determination the results confirm that their 
composition is similar to that of the younger Gower-Lidgbird lavas. 

Thus the highest flow exposed at Boat Harbour (BM.1966, P5, 42) has pyroxene 
with a mean 2V of 52^° as well as two generations of plagioclase. The groundmass 
crystals are mostly zoned with cores averaging An 50 and exteriors as sodic as An 35. 
The phenocrysts have a mean composition of An 72 and there may be some xeno- 
crysts since one crystal was found to be as calcic as An 82. The groundmasses of two 
other flows at this locality contained plagioclase of mean composition An 49 and An 
50. Only the lowest flow examined (BM.1966, P5, 46) about 200 feet below BM.1966, 
P5, 42) had small plagioclase grains averaging An 58 (An 53-64). With this one 
exception, therefore, the composition of the plagioclase accords exactly with the 
mean composition of the Gower-Lidgbird plagioclase and the optic axial angle of the 
pyroxene is also in good agreement with that of the phenocrystal pyroxene from the 
younger lavas. 

Therefore in spite of the long time interval which must have elapsed between the 
outpouring of these older lavas (expressed by their tilting and partial denudation), 
and the mid-Pliocene Gower-Lidgbird series there is no reason to think that they 
differed in composition in any material respect from the younger, fiat lying alkali 
olivine basalts which partly overly them. However the large proportion of coarse 
agglomerate in the inclined series bears witness to the fact that explosive volcanic 
activity was much more prevalent at this earlier epoch than it was in the later mid- 
Pliocene phase of vulcanism. 

Horizontal altered series. The older horizontal series also shows considerable 
alteration and the flows tend to have rubbly, lateritized upper portions — evidence 
of long periods of subaerial erosion. The few fairly fresh specimens collected were 
mainly from near sea level except at Malabar, where the summit (about 700 feet above 
sea level) is formed of a vesicular, porphyritic plagioclase-pyroxene-basalt, devoid 
of olivine, but with much granular ore altered to haematite (BM.1966, P5, 50). The 
phenocrysts occur in glomero-groups. In places there is a suggestion of flow structure. 
At Ned's beach a similar porphyritic basalt occurs, but here olivine is present, both as 
phenocrysts showing incipient iddingsitization, and in the groundmass where altera- 
tion to iddingsite is advanced. All specimens from this series contain a proportion 
of irresolvables, often green in colour. The fabric is usually intergranular. 

Determinations of the optic axial angle of the pyroxenes from the top of the series 



268 PETROLOGY OF LORD HOWE ISLAND, PART I 

(Malabar) gave a mean value of 53:|:° while a value of 53° was obtained from the base 
of the series, at the Old Gulch. The pyroxene is therefore presumed to be of uniform 
composition throughout the 700 feet of lavas here exposed. Moreover, as in the 
inclined series, this value of 2V exactly matches the mean value of the pyroxene 
phenocrysts from the younger Gower-Lidgbird series. Although no determinations 
have been made on the plagioclase, there seems no reason to doubt that the fresher 
basalts of the older horizontal series are similar to the Gower-Lidgbird basalts al- 
though they probably contain less oHvine. 

VII. BALL'S PYRAMID 

The dramatic spire of Ball's Pyramid (PL 14, i) over 1800 ft in height rises 
approximately 12 miles south of Mt Gower ; the sea between Mt Gower and Ball's 
Pyramid has a maximum depth of about 800 ft. The pyramid was visited on August 
13th 1965, but a relatively heavy swell and an extremely steep face allowed a brief 
landing only by one member of the party at considerable risk. 

The SW flank of the pyramid is sheer for 1800 ft. This stupendous cliff is con- 
siderably obscured by guano, but from a range of 20 to 30 yards the presence of 7 
dykes could be estabUshed. These are all relatively thin, almost vertical and their 
strike is approximately perpendicular to this South West face, i.e. the strike is 
roughly NE-SW. The dykes are about 20 yds apart. This spacing is the same as 
that on the north chff of Lord Howe Island, although the strike differs in the two 
localities. The lavas which they intersect are virtually horizontal, and appear to be 
considerably altered. These facts suggest that Ball's Pyramid does not belong 
to the Gower-Lidgbird series of younger volcanics, but to an older eruptive period. 
It is probable that the pyramid formed part of the older horizontal series, which 
builds the northern part of Lord Howe Island. The dyke rock collected from the 
south east corner of the pyramid (BM.1966, P5, 105) shows, in thin section (PI. 10, 2) 
glomero-groups of labradorite (mean composition An 62), but no xenocrysts of a 
more calcic composition. The groundmass is composed of fresh plagioclase laths 
(mean composition An 54), sparse pyroxene grains and a large proportion (probably 
50%) of a brownish semi-opaque product with much ore, and a pale yellow substance 
having an interior zone with aggregate polarization. Flow structure is weakly 
developed. 

VIII. AGE OF THE ISLAND 

It has been previously mentioned that the Gower-Lidgbird lavas form the youngest 
volcanic group on the island, since these flows are not cut by the dyke swarms which 
intersect the older lavas. The horizontal altered series is relatively older than the 
Gower-Lidgbird sequence, and although the age gap cannot be established it may not 
be as great as considered by David (1889). The Boat Harbour dipping lavas are of 
considerably greater antiquity than either of the horizontal series. Two agglomerate 
masses that were examined at the south end of Blinkenthorpe beach (PI. 14, 2) 
and NW of Mt Lidgbird have concealed field relationships and it is impossible to 
establish their positions in the relative age groups. Since they are intersected by 
altered sheets and dykes they probably belong to an initial phase in the volcanic 



PETROLOGY OF LORD HOWE ISLAND, PART I 269 

history of the island. Fortunately the Gower-Lidgbird lavas of Lord Howe Island 
are sufficiently fresh for K/Ar dating. In view of the fact that some contain a little 
devitrified glass it was felt that the most reliable ages would be those determined 
from the plagioclase, and not from whole-rock samples. The fresh plagioclase from 
the analysed oceanite (BM.1966, P5, 8) 2200 ft above sea level was separated ; its 
K2O content determined by flame photometry and its K/Ar ratio determined by 
J. A. Miller and F. J. Fitch. The results of two runs are given in Table XL 

Table XI 
Determination of age of oceanite from Gower-Lidgbird series 

K2O Air correction Vol- radiogenic "^QAr (mm^) Age and Error 

(wt. per cent) (per cent) Wt. of sample (gms) m.y. 

0'96 82-6 2-48x10"* 7-75±o-i8 

— 85-2 2'43Xio"* 7'6o±o-i7 

This younger volcanic series in the island is therefore of mid-Pliocene age. An 
attempt was made to date the horizontal altered series from separated plagioclase. 
This age was of the same order as that for the Gower-Lidgbird sequence. The results 
must be regarded as very approximate (error of ±2-5 m.y.) since the plagioclase is 
slightly altered and the amount that could be separated (0-03 gm) was extremely 
small. It is unfortunate that the inclined altered series contains no feldspar fresh 
enough for an age determination. The youngest alkali oUvine basalts of Lord Howe 
Island are, therefore, intermediate in age between the nearest lavas from NE New 
South Wales (McDougall and Wilkinson, 1967), where the youngest volcanics have 
ages of the order of 13-5 to 15 m.y. and the extensive Victorian " new " volcanic 
series, which mainly consists of alkaU olivine basalt with ages ranging from 4-5 to 
0.56 m.y. (McDougall et al., 1966). In New Zealand the North Island volcanics are 
mainly Recent, but the Lyttleton group of Banks Peninsula in South Island has been 
tentatively ascribed to the Pliocene (Curtis, 1965), although no radiogenic dating 
appears to have been made. This suite consists mainly of andesites and basalts ; 
olivine basalt flows are extremely rare. In the Dunedin field (Benson, 1946) the 
only two lavas that have been dated by radiogenic means (Curtis, 1965) are a tra- 
chyte from the initial eruptive phase, with an age of 15 m.y. and a phonolite from the 
third (and final) phase, which has an age of 11 -i m.y. 

In the Hawaiian chain (McDougall, 1964) the Mauna Kuwale trachyte, which 
outcrops on Oahu, has a mid-PUocene age (8-36 m.y.) but, with this one exception, 
the lavas from the rest of this chain are younger ; the oldest occurs on Kauai and 
gives a mean value of 5-62 m.y. 

The inchned, altered series of lavas and agglomerates is evidence of an additional 
much earher phase of volcanic activity on the present site of Lord Howe Island. 
As mentioned earlier the Gower-Lidgbird series (and the horizontal altered series 
forming the northern part of the island) are virtually horizontal over the whole 
area of their outcrops (PI. 13). This horizontahty over such a large surface imphes 

D 



270 



PETROLOGY OF LORD HOWE ISLAND, PART I 



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PETROLOGY OF LORD HOWE ISLAND, PART I 271 

the existence of an exceedingly liquid magma, probably of fissure eruption type, 
flowing over a level surface. It seems reasonable to suppose that the younger dyke 
swarm acted as feeders for the flows in view of the close similarities already noted 
in the minerals of both. In the south west (but not in the east) the inclined lavas 
and agglomerates were probably reduced to sea level before the younger flows covered 
them horizontally. If we accept Gass' (1967) assumption that ocean islands once they 
become inactive require from 20 to 25 million years for their reduction to atolls, the 
older lavas and agglomerates must have an age not less than about 30 million years, 
since it has been shown that their partial covering by the younger Gower-Lidgbird 
series took place at least 7 million years ago. It would therefore seem probable 
that volcanic activity on the present site of Lord Howe Island had started in the 
mid-Tertiary, and that the main product of this earher phase was also alkaU olivine 
basalt essentially similar to the Pliocene lavas. The younger volcanics of Lord Howe 
Island, formed about 8 million years ago, have on Gass' assumptions lost at least half 
their original volume, since the rate of decay is Ukely to be exponential. Mounts 
Gower and Lidgbird were probably more than twice their present heights, thus 
matching Recent cones such as Tahiti and Tristan da Cunha (about 7000 ft) ; conse- 
quently as much as 4000 ft of upper lavas may have been removed. The evidence 
points to a trachytic late differentiate which has probably been eroded from 
the summits of the two mountains. The younger volcanics that are preserved, after 
this extensive erosion, show only relatively shght differentiation, the latest product 
being a trachyandesite, with the possible exception of a thin, altered trachyte near the 
summit of Mt Gower. The older lavas have, naturally, suffered far greater erosion 
than the Gower-Lidgbird sequence. The latest bathymetrical chart of the Lord Howe 
Island area (Text-fig. 2), and the two sections drawn across this chart (Text-fig. 
21), show that the base of the original volcano measured approximately 30 miles from 
north to south by 10 miles from east to west. The present island (about 7 sq miles) 
consequently occupies only about one fortieth of its original area and a much smaller 
fraction of its original volume. In this connection it may be compared with Inacces- 
sible Island (in the Gough Island group), which, with a radiometrically determined 
age of 3 m.y. is considered by Gass (1967) to have lost 90 to 95% of its original volume 
and to have had a postulated original height of 7500 ft reduced to 1800 ft. 



IX. SUMMARY AND CONCLUSIONS 

The results of this investigation suggest the following conclusions. 

I. Lord Howe Island is composed of the following units : 
{a) Fresh alkali-basalts, mid-Pliocene according to radiometric age determinations, 

build Mounts Gower and Lidgbird at the southern end of the island. The flows 

are horizontal and the series is nearly 3000 ft thick. Its original thickness was 

probably twice as great. 
(6) An older horizontal, partly altered basaltic series (possibly of Pliocene age) forms 

the northern part of the island and probably also outcrops at Ball's Pyramid 

12 miles south of Mt Gower. 
(c) A series of inclined altered basaltic flows, outcrop in the SE of the island, where 



272 PETROLOGY OF LORD HOWE ISLAND, PART I 

at least a thousand feet are exposed. Near the base of this series coarse 
agglomerates are abundantly developed. 
{d) Dykes traverse each of the above groups with the exception of the youngest 
series (series a). Only the younger dykes, i.e. those that cut series b consist 
of fresh basaltic types. The remaining dyke-rocks show alteration in varying 
degrees. 

Detailed examination has been confined to the fresh rocks, namely the Gower- 
Lidgbird lavas and the younger dyke swarm. 

2. Considerable mineralogical and textural variations occur in the Gower- 
Lidgbird series, which includes both non-porphyritic lavas, and other lavas in which 
phenocrysts account for about one-quarter of the whole. Oceanites and oceanite- 
ankaramites form at least five different horizons and are believed, from chemical 
considerations, to have been formed by the accumulation of olivine and pyroxene 
in basaltic magma, without assimilation of lime or other material. 

3. Minor differentiation towards a trachytic residuum occurs towards the top 
of the Gower-Lidgbird series. At the top of this series the plagioclase is albitized. 

4. The younger dykes are fresh, usually narrow, steeply dipping, non-composite 
and show a tendency to be radial about Mt Lidgbird. In certain places the swarm 
is so dense that only remnants of the older lavas can be discerned between them. 

5. In the majority of these dykes oh vine is rare or absent ; about 85% of the 
dykes are porphyritic, having plagioclase as the commonest phenocryst. Some 
dykes contain up to 15% (modal) of primary, iron-rich magnesian carbonate. 
Many dykes have spherical vesicles infilled by deuteric minerals with carbonate 
always at the centre. In the rare olivine-bearing dykes olivine phenocrysts form up 
to 30% of the rock. In all the dykes ohvine is absent or sparse in the groundmass. 

6. The olivine bearing dykes preceded the non-ohvine types in order of intrusion. 

7. The average chemical composition of the fresh basalts (from both lavas and 
dykes) is similar to the average alkali andesites given by Nockolds (1954). Other 
basaltic types with comparable compositions occur in East Otago (New Zealand), 
central Victoria (Australia), Tutuila (Samoa) and the circum- Japan sea province. 

8. Vulcanism is considered to have been active for at least 25 million years, 
and to have ended in the mid-PUocene. The Lord Howe Rise is of considerable anti- 
quity (probably Paleozoic) . A low angle unconformity reported from the north east 
flank of the rise implies former elevation of the crest above sea level. This could 
be the source for the sediments that accumulated in the New Zealand Permian and 
early Mesozoic geosyncline. 

9. Throughout the period of vulcanicity there is no evidence of change in the 
overall composition of the lavas ; the older lavas (in spite of alteration) appear to 
have belonged to the alkali-basalt group. 

10. In the distribution of selected oxides used by Chayes (1964) to distinguish 
petrologically oceanic from petrologically circum-oceanic lavas. Lord Howe Island, 
though conforming closely to the oceanic group shows a slight displacement in 
each oxide towards the peak value for the circum-oceanic basalts. This may be 
connected with the position of Lord Howe Island in a Mediterranean type sea floor 
rather than in a true oceanic environment. 



PETROLOGY OF LORD HOWE ISLAND, PART I 273 

11. Lord Howe Island exemplifies the concept that Cainozoic volcanic islands are 
sparse off continental ocean borders not marked by a deep-sea trench, and that those 
which do occur are likely to be petrographically oceanic. 

12. The relation between the degree of silica undersaturation of the Lord Howe 
Island average basalt and its distance from the crest of the submarine ridge is similar 
to the relation between these two variables in the islands close to the Mid- Atlantic 
Ridge and the East Pacific Rise. 

13. The composition of the plagioclase phenocrysts from the Gower-Lidgbird 
lavas varies from about An 65 at the base of the series to about An 55 at the top. 
This variation is not regularly progressive but rhythmical. Xenocrysts with com- 
positions from An 70 to An 80 only occur at the base of the series. The groundmass 
plagioclase has an average composition of An 50 almost throughout the series, 
becoming slightly more sodic near the top. 

14. The larger plagioclase crystals of the dykes occur as : {a) phenocrysts, [b) 
xenocrysts, (c) spongy or checkerboard crystals and {d) crystals with oscillatory 
zoning. Variations in the values of 2V and of l oio'^a and l 010^^8 exceed experi- 
mental error, and are considered to be due to variations in the degree of ordering 
of the lattice. This implies differing thermal histories and suggests different depths 
of derivation for the phenocrysts. 

15. The fresh pyroxenes of both the Gower-Lidgbird volcanics and of the younger 
dyke rocks show no optical variations exceeding the experimental error. The 
phenocrysts have an average composition of Ca42Mg49Fe9 whilst the groundmass 
pyroxenes have a composition of Ca44Mg43Fe^3. In a zoned phenocryst electron- 
probe analysis gave a Mg : Fe ratio of 1-46 for the centre and i-2i for the margins. 
Two chemical analyses of phenocrysts and groundmass pyroxenes are given. The 
compositions, trend (parallel to the Di-Hd join), and the relation between pyroxene 
and the magma from which it separates (pyroxene is richer in lime) are all typical 
features of pyroxenes from alkali basalts. 

16. In the Gower-Lidgbird volcanics the olivine shows no significant variation 
in the flows of the upper 1300 ft where it averages 26% (mol) Fe2Si04. In the lower 
1500 ft (in which the flows are richer in olivine) it changes in composition from 18 to 
28% Fe2Si04 with a mean value of 23% Fe2Si04. Groundmass olivine has a compo- 
sition of 36% Fe2Si04. Ohvine phenocrysts from the dyke rocks, except those at 
North beach, have a mean composition of 22^% Fe2Si04. The North beach olivines 
are slightly, but significantly richer in magnesium with a mean composition of 18% 
Fe2Si04. 

17. Iron ores fall into two groups ; the first has a composition close to that of 
ilmenite, whilst the second approximates to a Ti-poor Fe-rich ulvospinel. The 
ilmenites of the dyke rocks have slightly more Fe and less Ti than do those of the 
lavas. The composition of the ulvospinel shows no difference as between dykes and 
flows. In the Gower-Lidgbird lava sequence there is (apart from a few exceptions) a 
definite trend from ilmenite bearing flows in the lower layers to ulvospinel bearing 
lavas near the top. 

18. 2V is of only secondary value as a guide to the composition of calcic plagio- 
clases. 



274 PETROLOGY OF LORD HOWE ISLAND. PART I 

19. The values of the cell sides and of the angle j8 are a more reliable guide to the 
composition of monoclinic pyroxenes than are their optical properties (refractive 
index and 2 V) . 

X. ACKNOWLEDGEMENTS 

I wish to record my most grateful thanks to Dr J. D. H. Wiseman for critically 
reading the MSS and for his invaluable advice and help. 

My thanks are due to Dr S. J. B. Reed and Mrs J. M. Hall for their electron 
microprobe analyses of ores, pyroxenes and plagioclases. The chemical analyses 
were done by Dr A. A. Moss and Mr J. Easton and to them also I am grateful. As 
already stated, K/Ar age determinations were done by Dr Miller and Mr Fitch 
whom I thank. I wish to thank Mr R. F. Symes and Miss E. Papworth for help with 
some of the pyroxene separations. The figures were drawn by Mr D. Moore to whom 
I record my gratitude. Finally I wish to put on record that Mr Clive Wilson of Lord 
Howe Island risked his life in heavy swell and a shark infested sea in order to obtain 
samples from the SW corner of Ball's Pyramid. 



XI. REFERENCES 

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160-170. 



PETROLOGY OF LORD HOWE ISLAND. PART I 275 

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Soc. Am. 15: 107-128. 
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276 PETROLOGY OF LORD HOWE ISLAND, PART I 

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724-743- 



P. M. Game, B.Sc. 
Department of Mineralogy 
British Museum (Natural History 
London, S.W.7 



PETROLOGY OF LORD HOWE ISLAND, PART I 



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PETROLOGY OF LORD HOWE ISLAND, PART I 



283 



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284 



PETROLOGY OF LORD HOWE ISLAND, PART I 



APPENDIX 2 

Petrography of analysed dyke rocks. 



Registration No. 



Type 



Locality 



Description 



BM.1966, P5, 64 alkali basalt North beach 



BM.1966, P5, 80 alkali basalt Ned's beach 



BM.1966, P5, 83 



alkali basalt 

with 

primary 

carbonate 



Blinkenthorpe 
beach 



BM.1966, P5, 102 



alkali 
olivine-basalt 



Admiralty 
Islets 



BM.1966, P5, 89 



alkali 
olivine-basalt 



Middle beach 



Plagioclase (relatively large grains) show 
partial alignment ; also occurs in glomero- 
groups. Somewhat smaller pyroxenes 
(larger ones show crystal outhnes) are set in 
a fine-grained assemblage of plagioclase, 
pyroxene, equidimensional iron ore, a little 
carbonate, very sparse iddingsitized olivine 
and a small amount of oUve coloured glass. 
Both plagioclase and pyroxene show evi- 
dence of strain and probably do not form 
two generations. Fabric tends to be inter- 
granular. There are some small, empty 
vesicles. 

A fine- and even-grained basalt, with par- 
tially aligned plagioclase laths, equidimen- 
sional grains of pyroxene (showing strain 
polarisation) , and some intergranular green- 
ish product showing aggregate polarisation. 
Devitrified glass is fairly abundant. Abun- 
dant ore consists entirely of ilmenite in 
" gridiron " form. 

Microphenocrysts or xenocrysts of plagio- 
clase (An 78-84) usually in glomero-groups, 
and often associated with pyroxene (of 
later consolidation) are set in an even- 
grained base of these minerals and skeletal 
ore. Olivine is absent. Primary iron rich 
carbonate is abundant (15% in the mode). 
There is a small proportion of irresolvable, 
interstitial residuum. There is no fluxion 
structure. 

Abundant, idiomorphic and subidiomorphic 
olivine phenocrysts showing partial cor- 
rosion and serpentinization along cracks ; 
less abundant, idiomorphic pale brownish- 
grey pyroxenes and sparse, small plagio- 
clase phenocrysts (An 63) and xenocrysts 
(An 72-77) lie in a plagioclase-pyroxene-ore 
groundmass, having a marked fluxion 
fabric. Intersertal pale green and colour- 
less, devitrified glass is fairly abundant. 
Ore occurs mainly as elongated shreds. 
An even-grained basalt with only sparse, 
small olivine microphenocrysts. The ma- 
trix consists of plagioclase laths showing a 
well marked ophitic relation to a purplish- 
brown pyroxene. Olivine, fresh or shghtly 
serpentinized is fairly abundant. Ore oc- 
curs mainly in elongated forms. There is a 
Httle olive-green, devitrified glass and some 
irresolvables. Sparse vesicles are fiUed by 
chabazite. There is no fluxion texture. 



I 



PLATE lo 

Fig. I . Vesicle filled by glass, plagioclase laths and semi-opaque products, with carbonate at 
centre. Plagioclase-basalt ; dyke, North beach ; ordinary light, x 15. BM.1966, P5, 30. 

Fig. 2. Plagioclase glomero-group. Plagioclase-basalt ; dyke, Ball's Pyramid ; crossed 
polars, X 15. BM.1966, P5, 105. 

Fig. 3. Plagioclase glomero-group. Basalt; dyke. North beach; crossed polars, X25. 
BM.1966, P5, 57. 

Fig. 4. Spongy plagioclase glomero-group. Plagioclase-basalt ; dyke. North beach ; 
crossed polars, x 20. BM.1966, P5, 56. 



Bull. Br. Mils. nat. Hist (Miner.) 2, 5 
I 



PLATE 10 










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PLATE II 

Fig. I. Plagioclase phenocryst, showing discontinuous zoning. Basalt ; dyke, Ned's 
beach ; crossed polars, x loo. BM.1966, P5, 76. 

Fig. 2. Ohvine phenocryst, partly serpentinised. Olivine-basalt ; dyke. North beach ; 
crossed polars, x 10. BM.1966, P5, 71. 

Fig. 3. Olivine phenocrysts with iddingsite rims. Alkali olivine-basalt ; flow, Gower- 
Lidgbird series ; 1800 ft above sea level, ordinary light, X 35. BM.1966, P5, 16. 

Fig. 4. Aligned plagioclase phenocrysts. Trachybasalt ; flow, Gower-Lidgbird series ; 
2450 ft above sea level ; crossed polars, X 15. BM.1966, P5, 4. 



Bull. Br. Mus. nat. Hist. (Miner.) 2, 5 



PLATE II 






'i 




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1 





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PLATE 12 

Fig. I. Olivine phenocrysts. Oceanite ; flow, Gower-Lidgbird series ; 500 ft above sea 
level ; ordinary light, x 12. BM.1966, P5, 36. 

Fig. 2. Well developed fluxion texture. Trachy-basalt ; inclined sheet, Gower-Lidgbird 
series ; 2300 ft above sea level ; crossed polars, x 125. BM.1966, P5, 6. 

Fig. 3. Twinned and slightly zoned pyroxene phenocryst. Olivine-pyroxene-basalt; 
dyke. Admiralty Islets ; crossed polars, x 20. BM.1966, P5, 102. 

Fig. 4. Pyroxene glomero-group. Alkali olivine-basalt ; flow, Gower-Lidgbird series ; 
330 ft above sea level ; crossed polars, x6o. BM.1966, P5, 39. 



Bull. Br. Mils. nat. Hist. (Miner.) 2, 5 
I 



PLATE 12 







PLATE 13 
Fig. I. Mt. Gower from the west. Height of cliff section about 2000 ft 
Fig. 2. Mt. Lidgbird from the north. Height of cliff section iioo ft 



Bull. Br. Mils. nat. Hist. (Miner.) 2, 5 



PLATE 13 



^^=?f^>> 






\ V —f 



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PLATE 14 
Fig. I. Ball's Pyramid from the north. Height of pyramid i860 ft 
Fig. 2. Agglomerate, cut by narrow dyke, at south end of Blinkenthorpe beach. 



Bull. Br. Mils, licit. Hist. (Miner.) 2, 5 



PLATE 14 




■4 






mmi 



PLATE 15 

Fig. I. Dense dyke swarm on north side of Rabbit Island. The dykes, most of which are 
m contact, dip steeply to the west. Height of section about 60 ft. 

Fig. 2. Inclined older lavas overlain by darker coloured wedge-shaped agglomerate mass; 
coast i mile north of Boat Harbour. 



Bull. Br. Mus. nat. Hist. (Miner.) 2, 5 



PLATE 15 






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